When you make the purchase it gives you a choice. Purchases put towards your bill MUST be in US dollars only. You can also opt to use your google checkout account or a credit card.

I've never used an android phone on T-Mobile service, but I do know that when I had one with Sprint there was never an option to add it to the bill, it was all done by card (that's what I used) to make any purchases. T-Mobile might be different though.