Taking SAT extra subject 2 without registration on the test day

For this coming Saturday’s test, I only registered for Biology E/M, but is it possible for me to take extra on test date?
I have heard from few friends of mine that it is possible, but I just wanted it to confirmed.

Thanks

Yes you can.

Absolutely can. Up to last year you were supposed to bring payment with you (a check) if you wanted to add a test or even two, but now they actually bill you. I would bring a check just in case.

YES!
I did this for the may exam. Basically, what I did was sign up for the Mathlvl2, but I took apush the day before so I took the US test as well. Just take the test, and they will email you your invoice.
It is kind of a pain, because there are no online payments. They will only let you pay by either calling customer service, faxing it, or sending a check by mail.
Good luck ;D