App Store credit
I have a credit in my AppStore account that I want to use to buy in itunes. Can I do this and how?
Just find what you want to buy and click on its price button to buy it, your balance should automatically be charged (as long as you aren't trying to gift) - if your account's balance covers the item's price (plus any applicable price) then only your balance will be charged, if it doesn't cover the price then the remainder will be charged to your credit card.