looks like u CAN use them like GC's
look
5. When prompted to select a payment method, enter your promotional
certificate claim code in the field provided on the order form.
6. If your order exceeds $10.00, you must pay for the balance of your
order with a credit card.
so you see, when you are going to pay you enter the giftcard number, but if total exceeds giftcard you have to pay by creditcard, but i havent tried stacking them yet, plese dont flame me if im wrong! =)