How to pay for an app using credit on my App Store card?
Likely the store is just trying to make sure you are the actual person with the account, and so it is asking for verification information rather than wanting to charge your credit card. If you have already entered credit card info in the past, re-entering it will not change any of Apple's data, but will verifiy that you are the same person who set up the account.

I beleive you can also hit the "none" button beside all the credit card icons if you do not have a credit card on file and are just relying on gift card credits.

http://support.apple.com/kb/ht2534