diff --git a/app/checkout/components/CheckoutContent.tsx b/app/checkout/components/CheckoutContent.tsx
index 5044414..f7f7a0f 100644
--- a/app/checkout/components/CheckoutContent.tsx
+++ b/app/checkout/components/CheckoutContent.tsx
@@ -20,6 +20,19 @@ import { readCheckoutDraft, writeCheckoutDraft, clearCheckoutDraft } from "../..
 import type { ShippingMethod, PaymentMethod, TrustBadge, ShippingSettings } from "../../lib/payload";
 import type { CustomerProfile } from "../../lib/customerAuth";
 
+// Native HTML5 pattern validation (instant, no round-trip) mirroring the
+// same rules Orders.ts/Customers.ts enforce server-side — a plausibility
+// check, not the source of truth (the backend re-validates regardless of
+// what a customer's browser did or didn't catch). Only Germany/Austria/
+// Switzerland are offered in the country <select>, so a fixed 3-way
+// lookup is enough — unlike the backend's own zip check, which stays
+// free-text-country-tolerant since Payload's admin has no such select.
+const PLZ_DIGITS: Record<string, number> = { Deutschland: 5, Österreich: 4, Schweiz: 4 };
+function plzPattern(country: string): string {
+  const digits = PLZ_DIGITS[country] ?? 4;
+  return `\\d{${digits}}`;
+}
+
 function FormField({
   label,
   wrapperClassName = "flex-1 min-w-0",
@@ -554,7 +567,19 @@ export function CheckoutContent({
               wrapperClassName="w-full sm:w-[calc(50%-0.5rem)] sm:flex-none min-w-0"
             />
             <div className="flex flex-col sm:flex-row gap-4 w-full">
-              <FormField label="PLZ" name="zip" type="text" value={zip} onChange={(e) => setZip(e.target.value)} placeholder="10115" autoComplete="postal-code" required />
+              <FormField
+                label="PLZ"
+                name="zip"
+                type="text"
+                value={zip}
+                onChange={(e) => setZip(e.target.value)}
+                placeholder="10115"
+                autoComplete="postal-code"
+                inputMode="numeric"
+                pattern={plzPattern(country)}
+                title={`PLZ muss aus ${PLZ_DIGITS[country] ?? 4} Ziffern bestehen.`}
+                required
+              />
               <FormField label="Ort" name="city" type="text" value={city} onChange={(e) => setCity(e.target.value)} placeholder="Berlin" autoComplete="address-level2" required />
             </div>
             <label className="flex flex-col gap-2 items-start w-full">
@@ -657,6 +682,9 @@ export function CheckoutContent({
                       inputMode="numeric"
                       placeholder="123"
                       autoComplete="off"
+                      pattern="\d{1,3}"
+                      maxLength={3}
+                      title="Packstationsnummer muss aus 1 bis 3 Ziffern bestehen (1–999)."
                       required
                     />
                     <FormField
@@ -665,8 +693,11 @@ export function CheckoutContent({
                       value={shippingPostNumber}
                       onChange={(e) => setShippingPostNumber(e.target.value)}
                       inputMode="numeric"
-                      placeholder="1234567"
+                      placeholder="1234567890"
                       autoComplete="off"
+                      pattern="\d{6,10}"
+                      maxLength={10}
+                      title="Postnummer muss aus 6 bis 10 Ziffern bestehen."
                       required
                     />
                   </div>
@@ -679,6 +710,9 @@ export function CheckoutContent({
                     onChange={(e) => setShippingZip(e.target.value)}
                     placeholder="10115"
                     autoComplete="off"
+                    inputMode="numeric"
+                    pattern={plzPattern(shippingCountry)}
+                    title={`PLZ muss aus ${PLZ_DIGITS[shippingCountry] ?? 4} Ziffern bestehen.`}
                     required
                   />
                   <FormField
diff --git a/package-lock.json b/package-lock.json
index e2a2835..ebac7af 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -359,7 +359,7 @@
     },
     "node_modules/@einfach-produktiv/invoicing": {
       "version": "0.1.0",
-      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#f6d8e9eaf55e3a6ce3ca4b8246d5a8ed3dac7d02",
+      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#6af96eb9b8adb3caf3fb610a37c1aeb788eca2f2",
       "dependencies": {
         "@e-invoice-eu/core": "^3.1.1"
       },