I would call USAA and talk with them. Normally Carnival will require you to go through the agency you booked the cruise with. So ask USAA what else you need to do.
It won't hurt to call Carnival and confirm their fax number. You could even ask their advice.
I wouldn't wait. Good luck and congrats on your anniversary! We are celebrating our 23rd year on the 3 Dec Elation cruise.