If you have a credit card account you don’t have to add just select item then cash out. If you really want to add money go buy an iTunes card.

I already figured it out, as gooch said, you pay as you buy.

When purchases are made a credit card on the account will have the money automatically taken out of the account, unless you have an allowance set up already.

But if you want to add money to the account like a giftcard just go to the main store page and hit redeem for a gift card.