Q: Help using iTunes Credit balance

What exactly happens when you try to buy something (what prompts/messages show), and is this the first time that you've tried to buy something on the account ? You can't change how items are billed, your balance should be automatically used (unless gifting, that can only be done via a credit card), credit card details can be asked for on the first purchase on an account depending upon how the account was created, but it shouldn't be charged (except possibly a temporary store holding charge which your card issuer should remove within a few days or so).