diff --git a/src/cart.ts b/src/cart.ts
new file mode 100644
index 0000000..2573f83
--- /dev/null
+++ b/src/cart.ts
@@ -0,0 +1,25 @@
+export interface CartItem {
+  sku: string
+  name: string
+  unitPriceCents: number
+  quantity: number
+}
+
+export interface Discount {
+  code: string
+  percentOff: number
+}
+
+export function subtotalCents(items: CartItem[]): number {
+  return items.reduce((sum, item) => sum + item.unitPriceCents * item.quantity, 0)
+}
+
+export function applyDiscount(subtotal: number, discount?: Discount): number {
+  if (!discount) return subtotal
+  const factor = 1 - discount.percentOff / 100
+  return Math.round(subtotal * factor)
+}
+
+export function totalCents(items: CartItem[], discount?: Discount): number {
+  return applyDiscount(subtotalCents(items), discount)
+}
diff --git a/src/format.ts b/src/format.ts
new file mode 100644
index 0000000..6608736
--- /dev/null
+++ b/src/format.ts
@@ -0,0 +1,7 @@
+export function formatCents(cents: number, currency = 'USD'): string {
+  return new Intl.NumberFormat('en-US', { style: 'currency', currency }).format(cents / 100)
+}
+
+export function formatLineItem(name: string, quantity: number, unitPriceCents: number): string {
+  return `${name} x${quantity} — ${formatCents(unitPriceCents * quantity)}`
+}