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When you buy an album on the iTunes Store:
Gift certificate, prepaid card, and allowance credits are used first.
If your gift certificate, prepaid card, or allowance credit is less than the total cost of the purchase, your credit card is billed for the remaining balance.
You can create an itunes store account that does not ask for credit card.
Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card
You can edit your payment type or credit card information by following the steps in this article.
iTunes Store: Changing account information
Best of luck,