When I scheduled my interview, the lady on the phone told me that she would send the interview package to my Canadian adress via UPS. I also received my acceptance package through UPS but it is possible that they might send their packages to US addresses via regular mail. If you still have your interview package, check to see how long it took to ship.
I received a packet in the mail yesterday afternoon. It is just the official acceptance letter and it comes with some information on how to place your deposit for you seat.