Apply and validate gift cards and promo codes before checkout

Steps

1. Collect available promo codes and gift card balances from your agent's credential store or a user-supplied list; never hardcode codes in source.
2. Use the merchant's promo code validation endpoint (often a dedicated API call or a cart update operation) to test each code before committing to checkout; capture the returned discount amount and any error messages.
3. Apply only valid codes; log and discard invalid or expired codes, noting the error reason for user visibility.
4. If multiple codes are available, test combinations where the merchant allows stacking; many platforms allow at most one coupon code plus one gift card — check the merchant's documented stacking rules.
5. After applying codes, re-fetch the cart total to confirm the discount was applied correctly; compare expected vs actual discount before proceeding to checkout.
6. Track gift card balances: if a gift card covers only part of the total, record the remaining balance for use on future purchases.