I have taken the bus from Cancun to Chetumal quite a few times. Then I use a Belize transportation service to cross the border to Corozal. It costs under $30 for the ADO luxury bus, which is very comfortable. The Belize Menzies pick up service is about another $15 US. You have to set that up ahead of time, but Menzies is very reliable.
ADO also has a red eye bus from Cancun to Belize City. I do not know if it stops in Corozal, though. It is easier to go through the border crossing with a Menzies guide, than with a large bus group.