diff --git a/app/api/checkout/route.ts b/app/api/checkout/route.ts
index 9febdbb..8dd2d64 100644
--- a/app/api/checkout/route.ts
+++ b/app/api/checkout/route.ts
@@ -4,25 +4,11 @@ import { getShippingMethods, getPaymentMethods, getCompanySettings } from "../..
 import { validateDiscountCode, redeemDiscountCode } from "../../lib/discountServer";
 import { createOrder } from "../../lib/orderServer";
 import { getSessionCustomer, registerCustomer, setSessionCookie, type CustomerSummary } from "../../lib/customerAuth";
-import { fetchProductsBySlug, type RawProduct } from "../../lib/productsServer";
+import { fetchProductsBySlug } from "../../lib/productsServer";
+import { describeBundleContents } from "../../lib/bundleContents";
 import { sendCriticalAlert } from "../../lib/alertAdmin";
 import { sendOrderConfirmationEmail } from "../../lib/orderEmail";
 
-// A bundle's `bundleItems` is only ever resolved once, right here, into a
-// plain readable string — the order line stores this snapshot
-// (`items[].bundleContents`), not a structured sub-list, so a later change
-// to the bundle's own composition can never rewrite what a past order
-// actually contained. `product.bundleItems[].product` is a relationship
-// resolved by fetchProductsBySlug's depth:2 fetch — {id, name} objects
-// when populated, a bare id if depth somehow didn't reach it (skipped).
-function describeBundleContents(product: RawProduct): string | null {
-  if (!product.bundleItems || product.bundleItems.length === 0) return null;
-  return product.bundleItems
-    .map((line) => (typeof line.product === "object" ? `${line.quantity}× ${line.product.name}` : null))
-    .filter((s): s is string => Boolean(s))
-    .join(", ");
-}
-
 type CheckoutBody = {
   cart: CartItem[];
   shippingMethodId: number;
diff --git a/app/lib/__tests__/bundleContents.test.ts b/app/lib/__tests__/bundleContents.test.ts
new file mode 100644
index 0000000..c9ff817
--- /dev/null
+++ b/app/lib/__tests__/bundleContents.test.ts
@@ -0,0 +1,56 @@
+import { describe, it, expect } from "vitest";
+import { describeBundleContents } from "../bundleContents";
+import type { RawProduct } from "../productsServer";
+
+const product = (overrides: Partial<RawProduct> = {}): RawProduct => ({
+  id: 1,
+  slug: "starter-set",
+  name: "Starter-Set",
+  price: 29.9,
+  active: true,
+  image: null,
+  taxRatePercent: null,
+  bundleItems: null,
+  ...overrides,
+});
+
+describe("describeBundleContents", () => {
+  it("returns null for a regular (non-bundle) product", () => {
+    expect(describeBundleContents(product())).toBeNull();
+  });
+
+  it("returns null for an empty bundleItems array", () => {
+    expect(describeBundleContents(product({ bundleItems: [] }))).toBeNull();
+  });
+
+  it("formats a single bundle item as 'qty× name'", () => {
+    const result = describeBundleContents(
+      product({ bundleItems: [{ product: { id: 2, name: "ToDo-Karten" }, quantity: 2 }] }),
+    );
+    expect(result).toBe("2× ToDo-Karten");
+  });
+
+  it("joins multiple bundle items with a comma", () => {
+    const result = describeBundleContents(
+      product({
+        bundleItems: [
+          { product: { id: 2, name: "ToDo-Karten" }, quantity: 2 },
+          { product: { id: 3, name: "Wochenplaner" }, quantity: 1 },
+        ],
+      }),
+    );
+    expect(result).toBe("2× ToDo-Karten, 1× Wochenplaner");
+  });
+
+  it("skips a line whose product didn't resolve to an object (depth miss)", () => {
+    const result = describeBundleContents(
+      product({
+        bundleItems: [
+          { product: 5, quantity: 1 },
+          { product: { id: 3, name: "Wochenplaner" }, quantity: 1 },
+        ],
+      }),
+    );
+    expect(result).toBe("1× Wochenplaner");
+  });
+});
diff --git a/app/lib/__tests__/cartTotals.test.ts b/app/lib/__tests__/cartTotals.test.ts
new file mode 100644
index 0000000..32816b7
--- /dev/null
+++ b/app/lib/__tests__/cartTotals.test.ts
@@ -0,0 +1,65 @@
+import { describe, it, expect } from "vitest";
+import { computeSubtotal, computeCartTotals, type CartLine } from "../cartTotals";
+import type { Product } from "../payload";
+
+const product = (overrides: Partial<Product> = {}): Product => ({
+  id: "todo-karten",
+  name: "ToDo-Karten",
+  description: "",
+  price: 12.9,
+  compareAtPrice: null,
+  image: "",
+  href: null,
+  active: true,
+  updatedAt: new Date().toISOString(),
+  spotlight: false,
+  spotlightEyebrow: null,
+  spotlightHeadline: null,
+  spotlightText: null,
+  spotlightImage: null,
+  ...overrides,
+});
+
+const line = (qty: number, productOverrides: Partial<Product> = {}): CartLine => ({ entry: { qty }, product: product(productOverrides) });
+
+describe("computeSubtotal", () => {
+  it("sums quantity × price across lines", () => {
+    expect(computeSubtotal([line(2, { price: 10 }), line(1, { price: 5 })])).toBe(25);
+  });
+
+  it("returns 0 for an empty cart", () => {
+    expect(computeSubtotal([])).toBe(0);
+  });
+});
+
+describe("computeCartTotals", () => {
+  it("adds shipping on top of the subtotal with no discount", () => {
+    const totals = computeCartTotals([line(1, { price: 20 })], 2.9, null);
+    expect(totals.subtotal).toBe(20);
+    expect(totals.total).toBeCloseTo(22.9, 6);
+    expect(totals.discountAmount).toBe(0);
+  });
+
+  it("applies a percent discount before adding shipping", () => {
+    const totals = computeCartTotals([line(1, { price: 100 })], 5, { type: "percent", value: 10 });
+    expect(totals.discountAmount).toBe(10);
+    expect(totals.total).toBe(95); // 100 - 10 + 5
+  });
+
+  it("applies a fixed discount, clamped so the total never goes negative", () => {
+    const totals = computeCartTotals([line(1, { price: 5 })], 0, { type: "fixed", value: 50 });
+    expect(totals.discountAmount).toBe(5); // clamped to subtotal
+    expect(totals.total).toBe(0);
+  });
+
+  it("computes totalSavings from compareAtPrice, separately from the discount code", () => {
+    const totals = computeCartTotals([line(2, { price: 10, compareAtPrice: 15 })], 0, null);
+    expect(totals.totalSavings).toBe(10); // 2 × (15 - 10)
+    expect(totals.subtotal).toBe(20); // uses price, not compareAtPrice
+  });
+
+  it("ignores compareAtPrice when it isn't actually higher than price", () => {
+    const totals = computeCartTotals([line(1, { price: 10, compareAtPrice: 10 })], 0, null);
+    expect(totals.totalSavings).toBe(0);
+  });
+});
diff --git a/app/lib/__tests__/invoicePdf.test.ts b/app/lib/__tests__/invoicePdf.test.ts
new file mode 100644
index 0000000..5c58a66
--- /dev/null
+++ b/app/lib/__tests__/invoicePdf.test.ts
@@ -0,0 +1,97 @@
+import { describe, it, expect } from "vitest";
+import { __testables, type InvoiceItem, type InvoiceOrder } from "../invoicePdf";
+
+const { isPaidImmediately, groupByTaxRate } = __testables;
+
+const item = (overrides: Partial<InvoiceItem> = {}): InvoiceItem => ({
+  productName: "ToDo-Karten",
+  quantity: 2,
+  unitPrice: 12.9,
+  taxRatePercent: 19,
+  bundleContents: null,
+  ...overrides,
+});
+
+const order = (overrides: Partial<InvoiceOrder> = {}): InvoiceOrder => ({
+  orderNumber: "#EP-0001",
+  invoiceNumber: "RE-0001",
+  invoiceIssuedAt: new Date().toISOString(),
+  customerFirstName: "Max",
+  customerLastName: "Mustermann",
+  deliveryMethod: "address",
+  street: "Musterweg 1",
+  zip: "10115",
+  city: "Berlin",
+  country: "Deutschland",
+  paymentMethodTitle: "Kreditkarte",
+  items: [item()],
+  subtotal: 25.8,
+  shippingCost: 2.9,
+  discountAmount: 0,
+  discountCode: null,
+  total: 28.7,
+  ...overrides,
+});
+
+describe("isPaidImmediately", () => {
+  it("is true for Kreditkarte", () => {
+    expect(isPaidImmediately("Kreditkarte")).toBe(true);
+  });
+
+  it("is true for PayPal", () => {
+    expect(isPaidImmediately("PayPal")).toBe(true);
+  });
+
+  it("is false only for Überweisung", () => {
+    expect(isPaidImmediately("Überweisung")).toBe(false);
+  });
+
+  it("defaults to true for any future/unknown payment method (only Überweisung is the named exception)", () => {
+    expect(isPaidImmediately("Sofortüberweisung")).toBe(true);
+    expect(isPaidImmediately("Klarna")).toBe(true);
+  });
+});
+
+describe("groupByTaxRate (original invoice)", () => {
+  it("reconciles net+tax to gross, and gross to the order total for a single rate", () => {
+    const o = order({ items: [item({ quantity: 2, unitPrice: 12.9 })], subtotal: 25.8, shippingCost: 2.9, discountAmount: 0, total: 28.7 });
+    const groups = groupByTaxRate(o, 19);
+    expect(groups).toHaveLength(1);
+    const [g] = groups;
+    expect(g.net + g.tax).toBeCloseTo(g.gross, 6);
+    expect(g.gross).toBeCloseTo(28.7, 2);
+  });
+
+  it("distributes a discount proportionally, still reconciling to the discounted total", () => {
+    const o = order({
+      items: [item({ quantity: 1, unitPrice: 50 })],
+      subtotal: 50,
+      shippingCost: 0,
+      discountAmount: 10,
+      total: 40,
+    });
+    const groups = groupByTaxRate(o, 19);
+    const total = groups.reduce((sum, g) => sum + g.gross, 0);
+    expect(total).toBeCloseTo(40, 2);
+  });
+
+  it("splits multiple tax rates into separate groups that each reconcile", () => {
+    const o = order({
+      items: [item({ quantity: 1, unitPrice: 20, taxRatePercent: 19 }), item({ quantity: 1, unitPrice: 10, taxRatePercent: 7 })],
+      subtotal: 30,
+      shippingCost: 0,
+      discountAmount: 0,
+      total: 30,
+    });
+    const groups = groupByTaxRate(o, 19);
+    expect(groups).toHaveLength(2);
+    for (const g of groups) expect(g.net + g.tax).toBeCloseTo(g.gross, 6);
+    expect(groups.reduce((sum, g) => sum + g.gross, 0)).toBeCloseTo(30, 2);
+  });
+
+  it("falls back to the tenant default rate when an item has no taxRatePercent", () => {
+    const o = order({ items: [item({ taxRatePercent: undefined as unknown as number })] });
+    const groups = groupByTaxRate(o, 7);
+    expect(groups[0].rate).toBe(7);
+  });
+});
diff --git a/app/lib/bundleContents.ts b/app/lib/bundleContents.ts
new file mode 100644
index 0000000..1183efc
--- /dev/null
+++ b/app/lib/bundleContents.ts
@@ -0,0 +1,21 @@
+import type { RawProduct } from "./productsServer";
+
+// A bundle's `bundleItems` is only ever resolved once, at checkout, into a
+// plain readable string — the order line stores this snapshot
+// (`items[].bundleContents`), not a structured sub-list, so a later change
+// to the bundle's own composition can never rewrite what a past order
+// actually contained. `product.bundleItems[].product` is a relationship
+// resolved by fetchProductsBySlug's depth:2 fetch — {id, name} objects
+// when populated, a bare id if depth somehow didn't reach it (skipped).
+//
+// Extracted out of app/api/checkout/route.ts into its own module (not
+// just kept as a local function there) so it's importable from a unit
+// test — Next.js route.ts files only allow HTTP-method + a few config
+// exports, not arbitrary named exports.
+export function describeBundleContents(product: RawProduct): string | null {
+  if (!product.bundleItems || product.bundleItems.length === 0) return null;
+  return product.bundleItems
+    .map((line) => (typeof line.product === "object" ? `${line.quantity}× ${line.product.name}` : null))
+    .filter((s): s is string => Boolean(s))
+    .join(", ");
+}
diff --git a/app/lib/invoicePdf.tsx b/app/lib/invoicePdf.tsx
index 59a4838..40f7938 100644
--- a/app/lib/invoicePdf.tsx
+++ b/app/lib/invoicePdf.tsx
@@ -316,3 +316,8 @@ export function InvoiceDocument({ order, seller }: { order: InvoiceOrder; seller
 export async function renderInvoicePdf(order: InvoiceOrder, seller: InvoiceSeller): Promise<Buffer> {
   return renderToBuffer(<InvoiceDocument order={order} seller={seller} />);
 }
+
+// Exported for unit testing (see app/lib/__tests__/invoicePdf.test.ts) —
+// the actual money math and payment-status logic, independent of PDF
+// rendering.
+export const __testables = { isPaidImmediately, groupByTaxRate };
diff --git a/package-lock.json b/package-lock.json
index 448803a..53c1065 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -25,7 +25,8 @@
         "eslint": "^9",
         "eslint-config-next": "16.2.9",
         "tailwindcss": "^4",
-        "typescript": "^5"
+        "typescript": "^5",
+        "vitest": "^4.1.10"
       }
     },
     "node_modules/@alloc/quick-lru": {
@@ -1344,6 +1345,16 @@
         "node": ">=12.4.0"
       }
     },
+    "node_modules/@oxc-project/types": {
+      "version": "0.139.0",
+      "resolved": "https://registry.npmjs.org/@oxc-project/types/-/types-0.139.0.tgz",
+      "integrity": "sha512-r9gHphtCs+1M7J0pw6Sn/hh/Wpa/iQrOOkrNAlVLF/gHq+/CJmHIWKKUUhdWjcD6CIa8idarspCsASiXCXvFUw==",
+      "dev": true,
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/Boshen"
+      }
+    },
     "node_modules/@payloadcms/live-preview": {
       "version": "3.85.2",
       "resolved": "https://registry.npmjs.org/@payloadcms/live-preview/-/live-preview-3.85.2.tgz",
@@ -1540,6 +1551,311 @@
         "@react-pdf/stylesheet": "^6.2.1"
       }
     },
+    "node_modules/@rolldown/binding-android-arm64": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-android-arm64/-/binding-android-arm64-1.1.5.tgz",
+      "integrity": "sha512-lZg8fqIv2v7FF237bwMgzGZEJvGL79/s5knJ/i6FmsGF4XXlzccZ4jb+TrFIxtSSxFtIpdsgrPZeMk1I9AFcyQ==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "android"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-darwin-arm64": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-darwin-arm64/-/binding-darwin-arm64-1.1.5.tgz",
+      "integrity": "sha512-51Bnx9pNiMRKSUNtBfySkNJ9vMU9Hh3I1ozDd6gyPPYzaXCfnptUcEZxXGYFn+ul2dtcMUiqGR1Yai2K10uoTw==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-darwin-x64": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-darwin-x64/-/binding-darwin-x64-1.1.5.tgz",
+      "integrity": "sha512-Tm+gbfC0aHu1tBA/JvKQh32S0K6YgCHkiAF4/W6xX0K0RmNuc94VeK419dJoE65R5aRxmo+noZQSWrAMF6yb6g==",
+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-freebsd-x64": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-freebsd-x64/-/binding-freebsd-x64-1.1.5.tgz",
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+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "freebsd"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-linux-arm-gnueabihf": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-arm-gnueabihf/-/binding-linux-arm-gnueabihf-1.1.5.tgz",
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+      "cpu": [
+        "arm"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ],
+      "engines": {
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+      }
+    },
+    "node_modules/@rolldown/binding-linux-arm64-gnu": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-arm64-gnu/-/binding-linux-arm64-gnu-1.1.5.tgz",
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+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
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+      }
+    },
+    "node_modules/@rolldown/binding-linux-arm64-musl": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-arm64-musl/-/binding-linux-arm64-musl-1.1.5.tgz",
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+        "arm64"
+      ],
+      "dev": true,
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+      ],
+      "license": "MIT",
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+      }
+    },
+    "node_modules/@rolldown/binding-linux-ppc64-gnu": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-ppc64-gnu/-/binding-linux-ppc64-gnu-1.1.5.tgz",
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+      "cpu": [
+        "ppc64"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-linux-s390x-gnu": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-s390x-gnu/-/binding-linux-s390x-gnu-1.1.5.tgz",
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+        "s390x"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
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+      ],
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+      }
+    },
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+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-x64-gnu/-/binding-linux-x64-gnu-1.1.5.tgz",
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+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
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+      ],
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+      }
+    },
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+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "libc": [
+        "musl"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
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+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-openharmony-arm64": {
+      "version": "1.1.5",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-openharmony-arm64/-/binding-openharmony-arm64-1.1.5.tgz",
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+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
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@@ -6673,6 +7244,13 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/siginfo": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/siginfo/-/siginfo-2.0.0.tgz",
+      "integrity": "sha512-ybx0WO1/8bSBLEWXZvEd7gMW3Sn3JFlW3TvX1nREbDLRNQNaeNN8WK0meBwPdAaOI7TtRRRJn/Es1zhrrCHu7g==",
+      "dev": true,
+      "license": "ISC"
+    },
     "node_modules/source-map-js": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",
@@ -6689,6 +7267,20 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/stackback": {
+      "version": "0.0.2",
+      "resolved": "https://registry.npmjs.org/stackback/-/stackback-0.0.2.tgz",
+      "integrity": "sha512-1XMJE5fQo1jGH6Y/7ebnwPOBEkIEnT4QF32d5R1+VXdXveM0IBMJt8zfaxX1P3QhVwrYe+576+jkANtSS2mBbw==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/std-env": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/std-env/-/std-env-4.2.0.tgz",
+      "integrity": "sha512-oCUKSupKTHX53EyjDtuZQ64pjLJ6yYCtpmEw0goYxtjG9KpbRe8KAsl2tBUGU9DyMcJ0RwJ8GqJAFzMXcXW1Rw==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/stop-iteration-iterator": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/stop-iteration-iterator/-/stop-iteration-iterator-1.1.0.tgz",
@@ -6931,6 +7523,23 @@
       "integrity": "sha512-pkY1fj1cKHb2seWDy0B16HeWyczlJA9/WW3u3c4z/NiWDsO3DOU5D7nhTLE9CF0yXv/QZFY7sEJmj24dK+Rrqw==",
       "license": "MIT"
     },
+    "node_modules/tinybench": {
+      "version": "2.9.0",
+      "resolved": "https://registry.npmjs.org/tinybench/-/tinybench-2.9.0.tgz",
+      "integrity": "sha512-0+DUvqWMValLmha6lr4kD8iAMK1HzV0/aKnCtWb9v9641TnP/MFb7Pc2bxoxQjTXAErryXVgUOfv2YqNllqGeg==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/tinyexec": {
+      "version": "1.2.4",
+      "resolved": "https://registry.npmjs.org/tinyexec/-/tinyexec-1.2.4.tgz",
+      "integrity": "sha512-SHf/r48b7vOrjve9PxJo3MN5v5yuyjHvdUcrQffT3WXMUfnGmHDVbC4k3sHJaJTgZCwpUplIaAo5ANtMyp3YHg==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/tinyglobby": {
       "version": "0.2.17",
       "resolved": "https://registry.npmjs.org/tinyglobby/-/tinyglobby-0.2.17.tgz",
@@ -6979,6 +7588,16 @@
         "url": "https://github.com/sponsors/jonschlinkert"
       }
     },
+    "node_modules/tinyrainbow": {
+      "version": "3.1.0",
+      "resolved": "https://registry.npmjs.org/tinyrainbow/-/tinyrainbow-3.1.0.tgz",
+      "integrity": "sha512-Bf+ILmBgretUrdJxzXM0SgXLZ3XfiaUuOj/IKQHuTXip+05Xn+uyEYdVg0kYDipTBcLrCVyUzAPz7QmArb0mmw==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=14.0.0"
+      }
+    },
     "node_modules/to-regex-range": {
       "version": "5.0.1",
       "resolved": "https://registry.npmjs.org/to-regex-range/-/to-regex-range-5.0.1.tgz",
@@ -7303,6 +7922,84 @@
       "integrity": "sha512-EPD5q1uXyFxJpCrLnCc1nHnq3gOa6DZBocAIiI2TaSCA7VCJ1UJDMagCzIkXNsUYfD1daK//LTEQ8xiIbrHtcw==",
       "license": "MIT"
     },
+    "node_modules/vite": {
+      "version": "8.1.5",
+      "resolved": "https://registry.npmjs.org/vite/-/vite-8.1.5.tgz",
+      "integrity": "sha512-7ULLwsCdYx/nRyrpiEwvqb5TFHrMVZyBt+rg/OAXT7rgj/z+DtTDyKFeLAdDkubDVDKD8jOsndmy7m55XcfUsw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "lightningcss": "^1.32.0",
+        "picomatch": "^4.0.5",
+        "postcss": "^8.5.17",
+        "rolldown": "~1.1.5",
+        "tinyglobby": "^0.2.17"
+      },
+      "bin": {
+        "vite": "bin/vite.js"
+      },
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      },
+      "funding": {
+        "url": "https://github.com/vitejs/vite?sponsor=1"
+      },
+      "optionalDependencies": {
+        "fsevents": "~2.3.3"
+      },
+      "peerDependencies": {
+        "@types/node": "^20.19.0 || >=22.12.0",
+        "@vitejs/devtools": "^0.3.0",
+        "esbuild": "^0.27.0 || ^0.28.0",
+        "jiti": ">=1.21.0",
+        "less": "^4.0.0",
+        "sass": "^1.70.0",
+        "sass-embedded": "^1.70.0",
+        "stylus": ">=0.54.8",
+        "sugarss": "^5.0.0",
+        "terser": "^5.16.0",
+        "tsx": "^4.8.1",
+        "yaml": "^2.4.2"
+      },
+      "peerDependenciesMeta": {
+        "@types/node": {
+          "optional": true
+        },
+        "@vitejs/devtools": {
+          "optional": true
+        },
+        "esbuild": {
+          "optional": true
+        },
+        "jiti": {
+          "optional": true
+        },
+        "less": {
+          "optional": true
+        },
+        "sass": {
+          "optional": true
+        },
+        "sass-embedded": {
+          "optional": true
+        },
+        "stylus": {
+          "optional": true
+        },
+        "sugarss": {
+          "optional": true
+        },
+        "terser": {
+          "optional": true
+        },
+        "tsx": {
+          "optional": true
+        },
+        "yaml": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/vite-compatible-readable-stream": {
       "version": "3.6.1",
       "resolved": "https://registry.npmjs.org/vite-compatible-readable-stream/-/vite-compatible-readable-stream-3.6.1.tgz",
@@ -7317,6 +8014,122 @@
         "node": ">= 6"
       }
     },
+    "node_modules/vite/node_modules/picomatch": {
+      "version": "4.0.5",
+      "resolved": "https://registry.npmjs.org/picomatch/-/picomatch-4.0.5.tgz",
+      "integrity": "sha512-RvwwcruNjI1ncT5xRakeyS9Lf8lcItv34KD+aif+VH9kduAyfYBipGh12274xtenIPZ119/R9BdTBa8gAwSh0A==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/jonschlinkert"
+      }
+    },
+    "node_modules/vitest": {
+      "version": "4.1.10",
+      "resolved": "https://registry.npmjs.org/vitest/-/vitest-4.1.10.tgz",
+      "integrity": "sha512-R9jUTe5S4Qb0HCd4TNqpC7oGcrMssMRGXLW80ubjWsW9VH5GF8y1Y0SFLY9AbqSk6nt0PnOx4H4WNJYZ13GUPw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@vitest/expect": "4.1.10",
+        "@vitest/mocker": "4.1.10",
+        "@vitest/pretty-format": "4.1.10",
+        "@vitest/runner": "4.1.10",
+        "@vitest/snapshot": "4.1.10",
+        "@vitest/spy": "4.1.10",
+        "@vitest/utils": "4.1.10",
+        "es-module-lexer": "^2.0.0",
+        "expect-type": "^1.3.0",
+        "magic-string": "^0.30.21",
+        "obug": "^2.1.1",
+        "pathe": "^2.0.3",
+        "picomatch": "^4.0.3",
+        "std-env": "^4.0.0-rc.1",
+        "tinybench": "^2.9.0",
+        "tinyexec": "^1.0.2",
+        "tinyglobby": "^0.2.15",
+        "tinyrainbow": "^3.1.0",
+        "vite": "^6.0.0 || ^7.0.0 || ^8.0.0",
+        "why-is-node-running": "^2.3.0"
+      },
+      "bin": {
+        "vitest": "vitest.mjs"
+      },
+      "engines": {
+        "node": "^20.0.0 || ^22.0.0 || >=24.0.0"
+      },
+      "funding": {
+        "url": "https://opencollective.com/vitest"
+      },
+      "peerDependencies": {
+        "@edge-runtime/vm": "*",
+        "@opentelemetry/api": "^1.9.0",
+        "@types/node": "^20.0.0 || ^22.0.0 || >=24.0.0",
+        "@vitest/browser-playwright": "4.1.10",
+        "@vitest/browser-preview": "4.1.10",
+        "@vitest/browser-webdriverio": "4.1.10",
+        "@vitest/coverage-istanbul": "4.1.10",
+        "@vitest/coverage-v8": "4.1.10",
+        "@vitest/ui": "4.1.10",
+        "happy-dom": "*",
+        "jsdom": "*",
+        "vite": "^6.0.0 || ^7.0.0 || ^8.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@edge-runtime/vm": {
+          "optional": true
+        },
+        "@opentelemetry/api": {
+          "optional": true
+        },
+        "@types/node": {
+          "optional": true
+        },
+        "@vitest/browser-playwright": {
+          "optional": true
+        },
+        "@vitest/browser-preview": {
+          "optional": true
+        },
+        "@vitest/browser-webdriverio": {
+          "optional": true
+        },
+        "@vitest/coverage-istanbul": {
+          "optional": true
+        },
+        "@vitest/coverage-v8": {
+          "optional": true
+        },
+        "@vitest/ui": {
+          "optional": true
+        },
+        "happy-dom": {
+          "optional": true
+        },
+        "jsdom": {
+          "optional": true
+        },
+        "vite": {
+          "optional": false
+        }
+      }
+    },
+    "node_modules/vitest/node_modules/picomatch": {
+      "version": "4.0.5",
+      "resolved": "https://registry.npmjs.org/picomatch/-/picomatch-4.0.5.tgz",
+      "integrity": "sha512-RvwwcruNjI1ncT5xRakeyS9Lf8lcItv34KD+aif+VH9kduAyfYBipGh12274xtenIPZ119/R9BdTBa8gAwSh0A==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/jonschlinkert"
+      }
+    },
     "node_modules/which": {
       "version": "2.0.2",
       "resolved": "https://registry.npmjs.org/which/-/which-2.0.2.tgz",
@@ -7422,6 +8235,23 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/why-is-node-running": {
+      "version": "2.3.0",
+      "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",
+      "integrity": "sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "siginfo": "^2.0.0",
+        "stackback": "0.0.2"
+      },
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/word-wrap": {
       "version": "1.2.5",
       "resolved": "https://registry.npmjs.org/word-wrap/-/word-wrap-1.2.5.tgz",
diff --git a/package.json b/package.json
index 16753c2..92c8d73 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,8 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test:unit": "vitest run"
   },
   "dependencies": {
     "@payloadcms/live-preview-react": "^3.85.2",
@@ -26,6 +27,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.2.9",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.1.10"
   }
 }
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..eb23d8c
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,13 @@
+import { defineConfig } from "vitest/config";
+
+// Pure-logic unit tests only (cart/discount math, invoice tax-rate
+// grouping, bundle-contents string building) — no jsdom, no Next.js
+// server runtime, no Payload/DB. `node` environment is enough since none
+// of this touches the DOM. See each app/lib/**/__tests__ file for what's
+// covered and why.
+export default defineConfig({
+  test: {
+    environment: "node",
+    include: ["app/**/__tests__/**/*.test.ts"],
+  },
+});