Merida to Belize City
Departs from Merida Terminal Alta Brisa daily at 9 pm with stops at Corozal and Orange Walk.
Fare to BC: MX$492
Trip time: 8:30 hours.
Belize City to Merida
Departs from BC bus terminal daily at 7:00 pm with stops at Orange Walk (8:30 pm), Corozal (9:30 pm), and the most important hospitals in Merida (ORAM, Juárez, Clínica Mérida, Star Médica). Arrives at Terminal Alta Brisa at 6:00 am.
Those fares are from about 6 months ago; I haven't checked if they are still current.