how to charge credit card rather than itunes balance?

You can't, if you have a balance on your account then that will be used up first.

You can try gifting the item to yourself (assuming that it's not an in-app purchase) - gifts can only be paid for by a credit card, you can't use your balance for it (I haven't tried it but I assume that gifting to yourself works).

Gifting : http://support.apple.com/kb/HT2736