Can you use the iTunes gift card to buy the cards on the new iOS5 cards app?
Cards uses your Apple ID to make purchases rather than your iTunes account, when I ordered a card it asked for my CCV, the security number on the back of my card registered with my Apple ID. You will find the order history in your Apple account and not in your iTunes account.

This is the same service as the one found in iPhoto, which uses the same procedure.