diff --git a/app/(dashboard)/sales/[id]/so-detail-client.tsx b/app/(dashboard)/sales/[id]/so-detail-client.tsx
index 232ba6fa..bc63c5e0 100644
--- a/app/(dashboard)/sales/[id]/so-detail-client.tsx
+++ b/app/(dashboard)/sales/[id]/so-detail-client.tsx
@@ -1007,6 +1007,12 @@ export function SoDetailClient({ order: so, warehouses, currencies, externalOrde
               </button>
             </span>
           )}
+          {/* audit-M-o2c: the paid→unpaid mismatch from deleting a payment on an
+              advanced-status order is recorded as a payment_status_mismatch WARNING
+              activity log (the durable, accurate signal). A read-time chip on
+              `!paidAt` alone can't tell "shipped on credit, never paid" from
+              "was paid then unpaid", so it isn't shown here; the existing
+              Paid / Part. Paid indicators cover the payment state. */}
 
           {canRefund && (
             <Button type="button" variant="outline" size="sm" onClick={() => setShowRefund(true)} disabled={isPending}>
diff --git a/app/actions/sales.ts b/app/actions/sales.ts
index b8f5c895..2e516f5d 100644
--- a/app/actions/sales.ts
+++ b/app/actions/sales.ts
@@ -47,6 +47,7 @@ import {
 } from '@/lib/domain/sales/sales-order-tax-validation'
 import { isExternalRefundIdUniqueConflict } from '@/lib/domain/sales/refund-idempotency'
 import { shouldWarnPaidWithoutInvoice } from '@/lib/domain/sales/paid-without-invoice'
+import { isPaymentStatusMismatch } from '@/lib/domain/sales/o2c-guards'
 import {
   cancelSalesOrderFulfillmentState,
   updateSalesOrderStatusUnderLock,
@@ -1245,12 +1246,21 @@ export async function applySalesOrderStatusTransition(
         orderNumber: true,
         externalOrderNumber: true,
         status: true,
+        archived: true,
         shipFromWarehouseId: true,
         shoppingLinks: { where: { connector: 'woocommerce' }, select: { id: true }, take: 1 },
         lines: { select: { id: true, productId: true, sku: true, qty: true } },
       },
     })
     if (!so) return { success: false, error: 'Order not found' }
+    // audit-M-o2c: an archived order is filed away — block MANUAL status edits.
+    // Automated data-pushes still apply: the WooCommerce force-sync
+    // (internalBypassToken) and the sessionless delivery cron (skipPermissionCheck)
+    // are carrier/source-of-truth signals, so they bypass this guard — otherwise an
+    // archived-but-shipped order could never auto-reach DELIVERED.
+    if (so.archived && !bypassPermission && !options?.skipPermissionCheck) {
+      return { success: false, error: 'This order is archived; unarchive it before changing its status.' }
+    }
 
     const transition = validateManualSalesOrderStatusTransition(so.status, targetStatus, {
       bypass: bypassPermission,
@@ -2390,6 +2400,8 @@ export async function deletePayment(paymentId: string, orderId: string): Promise
           externalOrderNumber: true,
           currency: true,
           totalForeign: true,
+          status: true,
+          paidAt: true,
         },
       })
       if (!so) return { error: 'Order not found' }
@@ -2401,6 +2413,7 @@ export async function deletePayment(paymentId: string, orderId: string): Promise
         return { error: 'Payment not found for this order' }
       }
       await tx.payment.delete({ where: { id: paymentId } })
+      let becameUnpaid = false
       if (!payment.refundId) {
         const remainingPayments = await tx.payment.findMany({
           where: { orderId, refundId: null },
@@ -2410,12 +2423,17 @@ export async function deletePayment(paymentId: string, orderId: string): Promise
           if (p.currency !== so.currency) return sum
           return sum + Number(p.amount)
         }, 0)
+        const stillFullyPaid = totalPaid >= Number(so.totalForeign) - 0.0001
+        // Only a genuine paid → not-paid transition is a mismatch. An order that
+        // was never fully paid (e.g. shipped on credit terms) isn't flagged just
+        // because a partial payment was removed.
+        becameUnpaid = so.paidAt !== null && !stillFullyPaid
         await tx.salesOrder.update({
           where: { id: orderId },
-          data: { paidAt: totalPaid >= Number(so.totalForeign) - 0.0001 ? undefined : null },
+          data: { paidAt: stillFullyPaid ? undefined : null },
         })
       }
-      return { so, payment: { refundId: payment.refundId, amount: Number(payment.amount), currency: payment.currency } }
+      return { so, becameUnpaid, payment: { refundId: payment.refundId, amount: Number(payment.amount), currency: payment.currency } }
     }, STOCK_TX_OPTIONS)
     if ('error' in txResult) return { success: false, error: txResult.error }
     if (!txResult.payment.refundId) {
@@ -2461,6 +2479,20 @@ export async function deletePayment(paymentId: string, orderId: string): Promise
       description: `Deleted payment from order ${getSalesOrderReference(txResult.so)}`,
       metadata: { orderNumber: getSalesOrderReference(txResult.so), paymentId },
     })
+    // audit-M-o2c: deleting the last payment clears paidAt but does not revert
+    // status — flag the mismatch when the order has already advanced past
+    // payment (shipped/completed) so it doesn't sit silently unpaid-but-shipped.
+    if (isPaymentStatusMismatch(txResult.so.status, txResult.becameUnpaid)) {
+      await logActivity({
+        entityType: 'SALES_ORDER',
+        entityId: orderId,
+        action: 'payment_status_mismatch',
+        tag: 'sales',
+        level: 'WARNING',
+        description: `Order ${getSalesOrderReference(txResult.so)} is ${txResult.so.status} but is no longer fully paid after deleting a payment. Review whether the status should be reverted.`,
+        metadata: { orderNumber: getSalesOrderReference(txResult.so), status: txResult.so.status, paymentId },
+      })
+    }
     return { success: true }
   } catch (e) {
     await logActivity({
diff --git a/docs/workflows.md b/docs/workflows.md
index 19e254ce..e24d2df6 100644
--- a/docs/workflows.md
+++ b/docs/workflows.md
@@ -41,6 +41,22 @@ state (e.g. cancelled or refunded after dispatch) between the poll's SHIPPED
 query and the under-lock write, the guard rejects the change and the cron logs a
 `delivery_status_skipped` warning instead of forcing it.
 
+**`COMPLETED` vs `DELIVERED`.** Both are manual, operator-set terminal-ish
+statuses reached from `SHIPPED` (and `COMPLETED → DELIVERED`). `DELIVERED` means
+the carrier confirmed delivery and can be set automatically by the delivery-status
+cron; `COMPLETED` is a manual "this order is done from our side" marker for
+businesses that don't track delivery (no automatic trigger sets it). Neither is
+forced by any sync; both still allow `PARTIALLY_REFUNDED`/`REFUNDED` afterwards.
+Use `DELIVERED` when delivery tracking is in play, `COMPLETED` otherwise.
+
+Manual status edits are rejected on **archived** orders (unarchive first);
+automated pushes (WooCommerce force-sync, the delivery-status cron) still apply.
+Deleting a payment that takes an already-paid order in an advanced status
+(`SHIPPED`/`COMPLETED`/`DELIVERED`/`PARTIALLY_REFUNDED`) back below fully-paid
+does not auto-revert the status but raises a `payment_status_mismatch` warning
+activity log so the operator can decide (it fires only on a genuine paid→unpaid
+transition, not for orders that were never fully paid, e.g. credit terms).
+
 ### Shipments
 
 | Status | Allowed next statuses | Notes |
diff --git a/lib/domain/sales/o2c-guards.ts b/lib/domain/sales/o2c-guards.ts
new file mode 100644
index 00000000..c056e572
--- /dev/null
+++ b/lib/domain/sales/o2c-guards.ts
@@ -0,0 +1,43 @@
+// ---------------------------------------------------------------------------
+// Order-to-cash guard predicates (audit-M-o2c)
+//
+// Small pure helpers for the order-to-cash medium-gap fixes, extracted so the
+// numeric/threshold logic is unit-testable without a database.
+// ---------------------------------------------------------------------------
+
+export type SalesOrderStatusName = string
+
+/** Fixed rounding slack for the cumulative-refund-vs-total check (a hair over a penny). */
+export const REFUND_TOTAL_EPSILON = 0.011
+
+/**
+ * Statuses that have advanced past payment. An order in one of these that is no
+ * longer fully paid (e.g. after deleting its last payment) is a status/payment
+ * mismatch worth surfacing.
+ */
+export const PAID_EXPECTED_SALES_STATUSES: ReadonlySet<string> = new Set([
+  'SHIPPED',
+  'COMPLETED',
+  'DELIVERED',
+  'PARTIALLY_REFUNDED',
+])
+
+/**
+ * Whether recording `thisRefund` on top of `previouslyRefunded` would push the
+ * cumulative refunded amount past the order total (within a fixed rounding
+ * epsilon). Cumulative — not per-refund — so N partial refunds can't each creep
+ * over a relative slack.
+ */
+export function refundWouldExceedOrderTotal(
+  thisRefund: number,
+  previouslyRefunded: number,
+  orderTotal: number,
+  epsilon: number = REFUND_TOTAL_EPSILON,
+): boolean {
+  return thisRefund + previouslyRefunded > orderTotal + epsilon
+}
+
+/** Whether an order that just became not-fully-paid is in a status that expected payment. */
+export function isPaymentStatusMismatch(status: string, becameUnpaid: boolean): boolean {
+  return becameUnpaid && PAID_EXPECTED_SALES_STATUSES.has(status)
+}
diff --git a/lib/domain/sales/refund-service.ts b/lib/domain/sales/refund-service.ts
index 9440848c..74bc4c16 100644
--- a/lib/domain/sales/refund-service.ts
+++ b/lib/domain/sales/refund-service.ts
@@ -14,6 +14,7 @@ import { roundQuantity, toDecimal, type DecimalInput } from '@/lib/domain/math/d
 import { getSalesOrderReference } from '@/lib/sales-order-display'
 import { validateRefundSalesOrderStatusUpdate } from '@/lib/domain/workflows/action-guards'
 import { isFullRefundAmount } from '@/lib/domain/sales/refund-thresholds'
+import { refundWouldExceedOrderTotal } from '@/lib/domain/sales/o2c-guards'
 import {
   isStockMovementIdempotencyConflict,
   refundInboundMovementKey,
@@ -1342,7 +1343,10 @@ export async function createSalesOrderRefund(
       select: { totalBase: true },
     })
     const previouslyRefunded = existingRefunds.reduce((sum, refund) => sum + refundBoundaryNumber(refund.totalBase), 0)
-    if (totalBase + previouslyRefunded > refundBoundaryNumber(so.totalBase) * 1.001) {
+    // audit-M-o2c: cumulative refunded must not exceed the order total, with a
+    // fixed rounding epsilon (not a 0.1% relative slack, which on a large order
+    // is pounds of headroom) so N partial refunds can't creep over.
+    if (refundWouldExceedOrderTotal(totalBase, previouslyRefunded, refundBoundaryNumber(so.totalBase))) {
       return { error: 'Refund total would exceed order total' } as const
     }
 
diff --git a/tests/domain/sales/o2c-guards.test.ts b/tests/domain/sales/o2c-guards.test.ts
new file mode 100644
index 00000000..54bdb2a3
--- /dev/null
+++ b/tests/domain/sales/o2c-guards.test.ts
@@ -0,0 +1,43 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+
+import {
+  refundWouldExceedOrderTotal,
+  isPaymentStatusMismatch,
+  REFUND_TOTAL_EPSILON,
+} from '@/lib/domain/sales/o2c-guards'
+
+test('refund within the order total is allowed', () => {
+  assert.equal(refundWouldExceedOrderTotal(40, 50, 100), false)
+})
+
+test('refund exactly at the order total is allowed (epsilon slack)', () => {
+  assert.equal(refundWouldExceedOrderTotal(50, 50, 100), false)
+})
+
+test('single-shot full refund (no prior refunds) is always allowed', () => {
+  assert.equal(refundWouldExceedOrderTotal(100, 0, 100), false)
+  assert.equal(refundWouldExceedOrderTotal(9999.99, 0, 9999.99), false)
+})
+
+test('cumulative partial refunds cannot creep over the total (fixed epsilon, not relative)', () => {
+  // Old 0.1% relative slack on a £10,000 order allowed ~£10 over; the fixed
+  // epsilon rejects anything beyond a rounding penny.
+  assert.equal(refundWouldExceedOrderTotal(1, 10000, 10000), true)
+  // A sub-penny rounding remainder is still tolerated.
+  assert.equal(refundWouldExceedOrderTotal(0.01, 9999.99, 10000), false)
+  assert.ok(REFUND_TOTAL_EPSILON < 0.02)
+})
+
+test('isPaymentStatusMismatch: advanced status + became unpaid → mismatch', () => {
+  assert.equal(isPaymentStatusMismatch('SHIPPED', true), true)
+  assert.equal(isPaymentStatusMismatch('COMPLETED', true), true)
+  assert.equal(isPaymentStatusMismatch('DELIVERED', true), true)
+  assert.equal(isPaymentStatusMismatch('PARTIALLY_REFUNDED', true), true)
+})
+
+test('isPaymentStatusMismatch: pre-payment status or still-paid → no mismatch', () => {
+  assert.equal(isPaymentStatusMismatch('PROCESSING', true), false)
+  assert.equal(isPaymentStatusMismatch('ALLOCATED', true), false)
+  assert.equal(isPaymentStatusMismatch('SHIPPED', false), false)
+})