How to use iTune gift card?

Ok I have redeemed the code and US$ 20.00 now in my iTune account.

I tried to deal with in-app purchase. I mean I want to buy something in the game it's like a bundle US$ 19.99 but the iTune always said that I am insufficient fund and bring me to input credit card number which I set as none.

So I want to ask is this.

1) Do I still need credit card to buy in-app purchase even I have enough balance?

2) If I am input my Credit card in and make a purchase of US$ 19.99 will deduct from the balance not my credit card?

3) if there're a taxes, 20 US$ balance will be spent first then the rest charge my credit card?

4) Or I can't use balance to buy an in-app purchase at all?

Please help cuz my 20 US$ sinking in my iTune account unusable.