How can I use an iTunes credit (from a gift  card which has been entered) for a book burchase?

10-11  Source: Network gathering  Views:1 

I have entered a $25 gift card and would like to use it for a book purchase. When I ordered a book it was charged to my credit card not the iTunes credit.
iTunes Store: How to redeem a code - - If you have an iTunes Gift Card or other code that you want to redeem in the iTunes Store. Includes hierarchy of how things are charged.
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