How do I use gift card balance instead of credit card?

I opened my iTunes store account using a gift card but recently added a credit card in order to sign up for iTunes Match. At the time I had an existing balance on my iTunes card credit card. Upon adding the credit card to the account, I no longer see that balance as available and iTunes always charges my credit card for any purchases. I am NOT attempting to send gifts, just purchase apps and music. Is there any way to use my gift card in preference to the credit card? Must I remove the credit card from the account?