Support stacking multiple discount codes

src/discounts.ts

@@ -0,0 +1,10 @@
+import type { Discount } from './cart'
+
+/** Combine multiple discount codes into one effective factor. */
+export function applyDiscounts(subtotalCents: number, discounts: Discount[]): number {
+  let totalPercentOff = 0
+  for (const discount of discounts) {
+    totalPercentOff += discount.percentOff
+  }
+  return Math.round(subtotalCents * (1 - totalPercentOff / 100))
+}