The AeroCaribe service from Cancun to Belize City has been discontinued, at least for the time being (that service on various airlines comes and goes).
You can still bus to Chetumal on ADO,then take a Belize bus to Corozal or Belize City, but the whole process, while dirt cheap, eats up most of a day.
--Lan Sluder
Belize First Magazine
http://wwww.belizefirst.com/