Nick...when I deplane at Cancun you can board an ADO bus right outside the entrance to the airport before you even hit the parking lot.
Cash works the best either Canadian, US or pesos. Just be sure what bus you get on. As long as it's going South you can go to Playa Del Carmen, Tulum, Chetumal and into Belize.
Good Luck. The bus is comfortable and sometimes... too COLD.