Why aren't there multiple billing options - such as Gift Cards - with Family Share?

I set us up on Family Share when we purchased an iPad Mini for our daughter. I linked my wife's account and now every time she purchases something, it bills my credit card. I have a Gift Card credit tied to my account yet her purchases aren't being deducted from it first. I'm looking for information on how to add other credit cards to the account but Family Share doesn't seem to be able to do that which is lame. As an adult member, I want her own purchases to be billed to her own credit card or Gift Card. I'd like to set up my daughter's purchases with Gift Card credit as well. This is 2015; I can't believe these aren't available options with this service.