diff --git a/src/einvoice/buildEInvoiceData.ts b/src/einvoice/buildEInvoiceData.ts
index 3593590..77a006b 100644
--- a/src/einvoice/buildEInvoiceData.ts
+++ b/src/einvoice/buildEInvoiceData.ts
@@ -269,18 +269,36 @@ function invoiceLines(lines: ComputedLine[]): UblInvoice["cac:InvoiceLine"] {
   }) as unknown as UblInvoice["cac:InvoiceLine"];
 }
 
+// `payableCents` is the amount actually charged (the original invoice's
+// own stored `order.total`, or — for a correction — the natural total
+// this same breakdown derives, see the two call sites). It can differ
+// from this document's own internally-consistent TaxInclusiveAmount by a
+// cent, since `order.total` comes from checkout's own independent cart/
+// discount math, not this per-rate proportional allocation — EN16931 has
+// a dedicated field for exactly this gap, `cbc:PayableRoundingAmount`
+// (BT-114): BR-CO-16 requires PayableAmount = TaxInclusiveAmount -
+// PrepaidAmount + RoundingAmount, so the gap is declared explicitly here
+// rather than either silently forcing PayableAmount to disagree with the
+// actually-charged total, or forcing TaxInclusiveAmount to disagree with
+// its own correctly-summed parts (caught by Mustang's Phase 4 CI check —
+// BR-CO-16 flagged this as a hard error the moment PayableAmount and
+// TaxInclusiveAmount didn't match to the cent, not just the pre-existing
+// non-fatal "Arithmetical issue" warning this file's comments used to
+// (wrongly) assume was the full story).
 function legalMonetaryTotal(rateGroups: RateGroup[], payableCents: number): UblInvoice["cac:LegalMonetaryTotal"] {
   const lineExtensionCents = rateGroups.reduce((sum, g) => sum + g.lineNetCents, 0);
   const allowanceTotalCents = rateGroups.reduce((sum, g) => sum + g.allowanceCents, 0);
   const chargeTotalCents = rateGroups.reduce((sum, g) => sum + g.chargeCents, 0);
   const taxExclusiveCents = lineExtensionCents - allowanceTotalCents + chargeTotalCents;
   const taxInclusiveCents = taxExclusiveCents + rateGroups.reduce((sum, g) => sum + g.taxCents, 0);
+  const roundingCents = payableCents - taxInclusiveCents;
   return {
     ...amtCents("cbc:LineExtensionAmount", lineExtensionCents),
     ...(allowanceTotalCents > 0 ? amtCents("cbc:AllowanceTotalAmount", allowanceTotalCents) : {}),
     ...(chargeTotalCents > 0 ? amtCents("cbc:ChargeTotalAmount", chargeTotalCents) : {}),
     ...amtCents("cbc:TaxExclusiveAmount", taxExclusiveCents),
     ...amtCents("cbc:TaxInclusiveAmount", taxInclusiveCents),
+    ...(roundingCents !== 0 ? amtCents("cbc:PayableRoundingAmount", roundingCents) : {}),
     ...amtCents("cbc:PayableAmount", payableCents),
   } as unknown as UblInvoice["cac:LegalMonetaryTotal"];
 }
@@ -311,12 +329,10 @@ export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): I
       ...(allowanceChargeEntries ? { "cac:AllowanceCharge": allowanceChargeEntries } : {}),
       "cac:TaxTotal": taxTotal(rateGroups),
       // PayableAmount is the order's own stored `total` (what the customer
-      // was actually charged at checkout), not a re-derivation from this
-      // breakdown — the two can differ by a cent from independent rounding
-      // paths (this file's per-rate allocation vs. checkout's own cart
-      // math), which Mustang surfaces as a non-fatal arithmetic *warning*,
-      // not an error; the legally meaningful figure is what was actually
-      // charged.
+      // was actually charged at checkout) — any cent of drift against this
+      // breakdown's own TaxInclusiveAmount is declared via
+      // PayableRoundingAmount inside legalMonetaryTotal(), not silently
+      // absorbed into either figure.
       "cac:LegalMonetaryTotal": legalMonetaryTotal(rateGroups, Math.round(order.total * 100)),
       "cac:InvoiceLine": invoiceLines(lines),
     },