I have done this a few times now...your best bet and cost effectiveness, is first, go to a travel agency in cancun, I always go to the one closest to downtown on your left you can see it, take a bus to town. Anyways I always pickup my return airfare from Chetumal to cancun there..you probably can get both there and back, saves alot of time, I usually bus it to chetemal and like Keycocker said cab it to the border then to tropic air, but you can save 5 hours by flying, I always like at least flying back to Cancoun and soak up as much time in Belize, my email is
[email protected] if you have any more questions. Grant