Using App store credit

Sorry if this is in the wrong forum, or if it has already been asked before, my google-fu isn't coming up with a clear answer.

Basically I received a $10 gift card a few months ago for participating in a survery and I added it to my Apple ID. I bought an iPad last week and I've been downloading a few apps. It's set up with the same Apple ID as my iphone and the app store clearly shows "$10.00 credit". The problem is that the store doesn't seem to be using that credit when I purchase apps.

Is there something I'm supposed to do to tell it to use the credit balance instead of my credit card? Or is there something else I'm not aware of?

Thanks.