Similar to what happens with a restaurant gift card, they remain valid for your next transaction however, you cannot redeem them on the transaction that is already closed. The easiest way to think about it is if you had a restaurant coupon and you forget to give it to them. You complete your meal and then 2 weeks later you return to the restaurant wanting to redeem the coupon. Same situation here. The responsibility to redeem the coupon or gift c certificate is on the customer to present the gift certificate at the beginning of the transaction.