diff --git a/CHANGES.md b/CHANGES.md
index 9c38282..45e3317 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Add checkout() to the cart module: coupons (SAVE10, FLAT5), discount, tax and total.
 
 ## 1.2.0
 - total() rounds through roundCents().
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..8f515e9 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -8,3 +8,23 @@ export function subtotal(items) {
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
+
+export function checkout(items, { coupon } = {}) {
+  const sub = roundCents(subtotal(items.filter((item) => item.qty > 0)));
+  const code = (coupon ?? '').trim();
+  let discount = 0;
+  switch (code.toUpperCase()) {
+    case '':
+      break;
+    case 'SAVE10':
+      discount = roundCents(sub * 0.1);
+      break;
+    case 'FLAT5':
+      discount = Math.min(5, sub);
+      break;
+    default:
+      throw new Error(`Unknown coupon: ${code}`);
+  }
+  const tax = roundCents((sub - discount) * TAX_RATE);
+  return { subtotal: sub, discount, tax, total: roundCents(sub - discount + tax) };
+}
diff --git a/tests/lib/cart.test.js b/tests/lib/cart.test.js
index 7a9eab3..657055a 100644
--- a/tests/lib/cart.test.js
+++ b/tests/lib/cart.test.js
@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { subtotal, total } from '../../src/lib/cart.js';
+import { subtotal, total, checkout } from '../../src/lib/cart.js';
 
 const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
 
@@ -11,3 +11,26 @@ test('subtotal adds price times quantity', () => {
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('checkout without coupon', () => {
+  assert.deepEqual(checkout(items), { subtotal: 25, discount: 0, tax: 2, total: 27 });
+  assert.deepEqual(checkout(items, { coupon: '  ' }), checkout(items));
+});
+
+test('checkout ignores items with qty 0 or less', () => {
+  const r = checkout([...items, { price: 100, qty: 0 }, { price: 100, qty: -2 }]);
+  assert.equal(r.subtotal, 25);
+});
+
+test('checkout SAVE10 is case-insensitive and trimmed', () => {
+  assert.deepEqual(checkout(items, { coupon: ' save10 ' }), { subtotal: 25, discount: 2.5, tax: 1.8, total: 24.3 });
+});
+
+test('checkout FLAT5 takes $5 off, capped at subtotal', () => {
+  assert.deepEqual(checkout(items, { coupon: 'flat5' }), { subtotal: 25, discount: 5, tax: 1.6, total: 21.6 });
+  assert.deepEqual(checkout([{ price: 3, qty: 1 }], { coupon: 'FLAT5' }), { subtotal: 3, discount: 3, tax: 0, total: 0 });
+});
+
+test('checkout rejects unknown coupons with the trimmed code', () => {
+  assert.throws(() => checkout(items, { coupon: ' bogus ' }), { message: 'Unknown coupon: bogus' });
+});