Discount logic correctly applies percentage or fixed amounts

Incorrect discount arithmetic produces wrong prices in every context where the formula runs — cart totals, checkout confirmations, order emails, and refund calculations. The canonical bug: subtracting percentOff as a dollar amount instead of a fraction (charging $100 - 20 = $80 when the intent is $100 × 0.80 = $80 — those happen to be equal, masking the bug until 10% off $50 yields $50 - 10 = $40 instead of $45). A fixed discount that allows negative final prices means a $200 off coupon on a $150 product produces a -$50 cart that Stripe rejects or, worse, creates a refund. ISO 25010:2011 maintainability is the external mapping — calculation logic must be testable and auditable.

Implement all discount types in a single utility in src/lib/pricing.ts with explicit floor guards to prevent negative prices.

export const applyDiscount = (
  basePrice: number,
  discount: { type: 'percent' | 'fixed'; amount: number }
): number => {
  if (discount.type === 'percent') {
    return Math.max(0, basePrice * (1 - discount.amount / 100))
  }
  return Math.max(0, basePrice - discount.amount)
}

For stacked discounts, apply percentage discounts first, then fixed amounts — document the order explicitly in src/lib/pricing.ts so it doesn't become implicit tribal knowledge.