I know pretty much the only thing you can add to your AMCAS application after you submit is additional letters of recommendation... How late are they accepted and how do you send them in?
You can send them as late as you want to in the process (in fact, I sent a couple in late April to help in my waitlists). Just fill out the form for a LOR like you have done with the others and give your letter writer their AMCAS code. Once they send the letter in, AMCAS uploads it into your account and you can then individually add the letter to any school you applied to that participates in the LOR service. Good luck!