Q: Buying using Apple ID credit

I loaded a gift card a while back on my iTunes and now I would like to buy something with that preloaded credit, but I don't know how I would go about doing that.

I have tried just clicking buy and it says it requires billing info, when i go to fill it in I choose the option of redeem a gift card, then it asks me for the gift card code. I have thrown the card away because I didn't think it would be necessary to re-enter it if the money is on my apple ID already. Please tell me there is a way to do it without having to re-enter the code ***** not being able to use it.