iPad and iTune

I never had problem with using iTune card when purchasing from iTune. Recently, no matter what I do ( using iPad ), the purchase will be charged on my credit card, even when I have money, and it doesn't even give me a warning that it is going to be charged on my credit card, just a normal apple id password popup. Does iPad uses a different iTune card than what we use on our computers?
(Although this problem also happened last night when I used my laptop)

Thanks

The following Apple KB shows how to redeem a gift card and says that when you purchase something, your gift card, prepaid card and other credits or allowances are used first and if what you have covers the purchase price then it's done. If what you purchase costs more than the credits you have, then they use up whatever is there and the rest goes to the credit card.

It looks like it's an automatic thing without you having to explicitly say where to pull money from..

So you should confirm that the card/code you used was properly redeemed and you see the right credit in your account.To do that, open up iTunes or App Store and scroll to the bottom on the Featured tab on your iPad. If you have a balance, it should show up at the bottom. If you don't see anything just your Apple ID, then it might be that the redeem of the code didn't happen properly.