why does it keep charging my credit card when i have an itunes card on my account?

Are you a family organiser in family sharing ? If you are then you can't use your balance (unless you disband the family). If you aren't it should automatically use your balance, if it's not try contacting iTunes Support and see if they can see why it's happening : http://www.apple.com/support/itunes/ww or https://www.apple.com/emea/support/itunes/contact.html

If you don't know whether or not you've set up family sharing from your account, what shows on the account in Settings > iCloud on an iOS device - if that account isn't an organiser you would have the option to set up family sharing.