diff --git a/packages/db/src/billing-store.test.ts b/packages/db/src/billing-store.test.ts
index 8ed73b8..2647b6a 100644
--- a/packages/db/src/billing-store.test.ts
+++ b/packages/db/src/billing-store.test.ts
@@ -24,12 +24,12 @@ test("markInvoicePaid activates a pending invoice once and writes an admin audit
 
   assert.equal(result.status, "paid");
   assert.ok(result.paidAt instanceof Date);
-  assert.equal(database.calls.paymentInvoiceUpdate.length, 1);
+  assert.equal(database.calls.paymentInvoiceUpdateMany.length, 1);
   assert.equal(database.calls.subscriptionUpdate.length, 1);
   assert.equal(database.calls.usageLedgerCreate.length, 1);
   assert.equal(database.calls.adminAuditCreate.length, 1);
 
-  const paymentUpdate = database.calls.paymentInvoiceUpdate[0] as ({
+  const paymentUpdate = database.calls.paymentInvoiceUpdateMany[0] as ({
     data: { status: "paid"; paidAt: Date };
   } | undefined);
   const auditEntry = database.calls.adminAuditCreate[0];
@@ -75,7 +75,7 @@ test("markInvoicePaid is idempotent for already paid invoices", async () => {
 
   assert.equal(result.status, "paid");
   assert.equal(result.paidAt?.toISOString(), paidAt.toISOString());
-  assert.equal(database.calls.paymentInvoiceUpdate.length, 0);
+  assert.equal(database.calls.paymentInvoiceUpdateMany.length, 0);
   assert.equal(database.calls.subscriptionUpdate.length, 0);
   assert.equal(database.calls.usageLedgerCreate.length, 0);
   assert.equal(database.calls.adminAuditCreate.length, 1);
@@ -108,36 +108,100 @@ test("markInvoicePaid rejects invalid terminal invoice transitions", async () =>
       error.message === 'Invoice in status "expired" cannot be marked paid.',
   );
 
-  assert.equal(database.calls.paymentInvoiceUpdate.length, 0);
+  assert.equal(database.calls.paymentInvoiceUpdateMany.length, 0);
   assert.equal(database.calls.subscriptionUpdate.length, 0);
   assert.equal(database.calls.usageLedgerCreate.length, 0);
   assert.equal(database.calls.adminAuditCreate.length, 0);
 });
 
+test("markInvoicePaid treats a concurrent pending->paid race as a replay without duplicate side effects", async () => {
+  const paidAt = new Date("2026-03-10T12:00:00.000Z");
+  const database = createBillingDatabase({
+    invoice: createInvoiceFixture({
+      status: "pending",
+      paidAt: null,
+      subscription: createSubscriptionFixture(),
+    }),
+    updateManyCount: 0,
+    invoiceAfterFailedTransition: createInvoiceFixture({
+      status: "paid",
+      paidAt,
+      subscription: createSubscriptionFixture(),
+    }),
+  });
+
+  const store = createPrismaBillingStore(database.client);
+  const result = await store.markInvoicePaid({
+    invoiceId: "invoice_1",
+    actor: {
+      type: "web_admin",
+      ref: "admin_user_1",
+    },
+  });
+
+  assert.equal(result.status, "paid");
+  assert.equal(result.paidAt?.toISOString(), paidAt.toISOString());
+  assert.equal(database.calls.paymentInvoiceUpdateMany.length, 1);
+  assert.equal(database.calls.subscriptionUpdate.length, 0);
+  assert.equal(database.calls.usageLedgerCreate.length, 0);
+  assert.equal(database.calls.adminAuditCreate.length, 1);
+  assert.equal(database.calls.adminAuditCreate[0]?.action, "invoice_mark_paid_replayed");
+  assert.equal(database.calls.adminAuditCreate[0]?.metadata?.replayed, true);
+});
+
 function createBillingDatabase(input: {
   invoice: ReturnType<typeof createInvoiceFixture>;
+  updateManyCount?: number;
+  invoiceAfterFailedTransition?: ReturnType<typeof createInvoiceFixture>;
 }) {
   const calls = {
-    paymentInvoiceUpdate: [] as Array<Record<string, unknown>>,
+    paymentInvoiceUpdateMany: [] as Array<Record<string, unknown>>,
     subscriptionUpdate: [] as Array<Record<string, unknown>>,
     usageLedgerCreate: [] as Array<Record<string, unknown>>,
     adminAuditCreate: [] as Array<Record<string, any>>,
   };
 
   let currentInvoice = input.invoice;
+  let findUniqueCallCount = 0;
 
   const transaction = {
     paymentInvoice: {
-      findUnique: async () => currentInvoice,
-      update: async ({ data }: { data: { status: "paid"; paidAt: Date } }) => {
-        calls.paymentInvoiceUpdate.push({ data });
-        currentInvoice = {
-          ...currentInvoice,
-          status: data.status,
-          paidAt: data.paidAt,
-        };
+      findUnique: async () => {
+        findUniqueCallCount += 1;
+
+        if (
+          input.invoiceAfterFailedTransition &&
+          input.updateManyCount === 0 &&
+          findUniqueCallCount > 1
+        ) {
+          currentInvoice = input.invoiceAfterFailedTransition;
+        }
+
         return currentInvoice;
       },
+      updateMany: async ({
+        where,
+        data,
+      }: {
+        where: { id: string; status: "pending" };
+        data: { status: "paid"; paidAt: Date };
+      }) => {
+        calls.paymentInvoiceUpdateMany.push({ where, data });
+
+        const count =
+          input.updateManyCount ??
+          (currentInvoice.id === where.id && currentInvoice.status === where.status ? 1 : 0);
+
+        if (count > 0) {
+          currentInvoice = {
+            ...currentInvoice,
+            status: data.status,
+            paidAt: data.paidAt,
+          };
+        }
+
+        return { count };
+      },
     },
     subscription: {
       update: async ({ data }: { data: Record<string, unknown> }) => {
diff --git a/packages/db/src/billing-store.ts b/packages/db/src/billing-store.ts
index 5de59de..723a31c 100644
--- a/packages/db/src/billing-store.ts
+++ b/packages/db/src/billing-store.ts
@@ -170,14 +170,59 @@ export function createPrismaBillingStore(database: PrismaClient = defaultPrisma)
         }
 
         const paidAt = invoice.paidAt ?? new Date();
-        const updatedInvoice = await transaction.paymentInvoice.update({
-          where: { id: invoice.id },
+        const transitionResult = await transaction.paymentInvoice.updateMany({
+          where: {
+            id: invoice.id,
+            status: "pending",
+          },
           data: {
             status: "paid",
             paidAt,
           },
         });
 
+        if (transitionResult.count === 0) {
+          const currentInvoice = await transaction.paymentInvoice.findUnique({
+            where: { id: input.invoiceId },
+            include: {
+              subscription: {
+                include: {
+                  plan: true,
+                },
+              },
+            },
+          });
+
+          if (!currentInvoice) {
+            throw new BillingError("invoice_not_found", "Invoice not found.");
+          }
+
+          if (currentInvoice.status === "paid") {
+            await writeInvoicePaidAuditLog(transaction, currentInvoice, input.actor, true);
+            return mapInvoice(currentInvoice);
+          }
+
+          throw new BillingError(
+            "invoice_transition_not_allowed",
+            `Invoice in status "${currentInvoice.status}" cannot be marked paid.`,
+          );
+        }
+
+        const updatedInvoice = await transaction.paymentInvoice.findUnique({
+          where: { id: invoice.id },
+          include: {
+            subscription: {
+              include: {
+                plan: true,
+              },
+            },
+          },
+        });
+
+        if (!updatedInvoice) {
+          throw new BillingError("invoice_not_found", "Invoice not found.");
+        }
+
         if (invoice.subscription) {
           const periodStart = paidAt;
           const periodEnd = addDays(periodStart, 30);