Quantity-cap Discount Function

Scaffold the Discount Function

Start from Shopify CLI. Keep the Function tiny — CAP as a constant you can later move to metafields once the mechanic is proven on staging.

1// src/run.js
2const CAP = 3;
3
4export function run(input) {
5  const lines = input.cart.lines.filter((l) => l.quantity > 0);
6  if (lines.length === 0) return { discounts: [] };
7  return { discounts: lines.flatMap(capLine) };
8}

Split overflow to full price

When quantity exceeds CAP, discount only the capped units. Overflow becomes its own line at full price — the mechanic apps kept getting wrong.

1// src/cap.js
2function capLine(line) {
3  if (line.quantity <= CAP) {
4    return [{ targets: [{ cartLine: { id: line.id } }], value: { percentage: 20 } }];
5  }
6  // Only the first CAP units are discounted; the rest stay full price.
7  return [
8    {
9      targets: [{ cartLine: { id: line.id, quantity: CAP } }],
10      value: { percentage: 20 },
11    },
12  ];
13}

Verify edge cases before staging sign-off

Test mixed carts, gift lines, and CAP changes. If marketing can edit CAP mid-campaign, put it in a metafield — don't redeploy for a number.

• qty ≤ CAP → single discounted line
• qty > CAP → split + full-price overflow
• gift / zero-price lines untouched
• B2B company location pricing still resolves