Call HTC. I know for a fact that they took care of a billing error/question for me in less than 5 minutes when I called. Of course and email works as well.
I would call but it's big money for me too
Do like many of us do and set up a credit card/debit card payment for your account. If you dont have that option then mail a check to them or send a money order. HTC will work with you as best they can. Email them if you have a specific question. They will respond promptly.BigKev