On the test date, I remember seeing that billing was available after I take the test, but I just don’t know how to pay it and I am receiving any emails about how and what to pay through which means.
Does anybody know how to pay for the extra test that you decide to take on the test date?
Just in case for anybody facing the same issue, CollegeBoard does send an email (super late though, mine came 2 months after testing) telling you to fill out a form listing credit card serials, passwords, and security stuff! thats all. hope this would help 
@tiffym thanks! Was there any issue while receiving the scores of the extra test? Any delays?
We called CB and paid over the phone by credit card.
@samc24 sorry for the late reply, but my score (surprisingly) came out even before I was told to make a payment. No delays at all
They just charged my account automatically and I got an email receipt a few days afterward