Tax amount is calculated on correct subtotal (pre-discount vs. post-discount)

Applying a 10% coupon discount after tax rather than before it systematically overcharges US customers. In most US states, sales tax is calculated on the post-discount price — applying tax first and then the discount is incorrect under standard US sales tax rules (CWE-682, ISO 25010:2011 functional correctness). A $100 cart with a $10 coupon should produce $90 × 0.085 = $7.65 tax; applying tax before discount produces $8.50 — a $0.85 overcharge per transaction that compounds across every discounted order. When the discount/tax order differs between cart preview and checkout confirmation, customers see a lower total in the cart and a higher total at payment, which triggers abandonment and disputes.

Enforce the correct US order of operations in lib/checkout.ts: subtract discount from subtotal before computing tax:

// lib/checkout.ts — correct US: tax after discount
function calculateOrderTotal(
  items: OrderItem[],
  discountCode: string | undefined,
  address: Address
): OrderSummary {
  const subtotal = items.reduce((sum, i) => sum + i.price * i.quantity, 0)
  const discount = discountCode ? applyDiscount(subtotal, discountCode) : 0
  const subtotalAfterDiscount = subtotal - discount
  // Tax is computed on the post-discount taxable amount
  const tax = calculateTax(items, address, subtotalAfterDiscount)
  const shipping = calculateShipping(items, address)
  const total = subtotalAfterDiscount + tax + shipping
  return { subtotal, discount, tax, shipping, total }
}

Add a code comment citing the jurisdiction rule. If cart preview uses a separate path, consolidate both onto the same calculateOrderTotal function so the operation order is never divergent.