Crossing the border is no problem. It's usually pretty fast. Monday is probably the busiest day.
You'll go through Belize side, cross the bridge over the Rio Hondo and then go through Mexican customs and immigration on the Mexican side. I don't believe the Belize taxis are allowed to take you across the border. I know the Mexican ones just drop you on the Mexico side.
One option you might want to consider is to take a bus. Belize buses take you right through the border (letting you off and picking you up, about a buck from Corozal Town) and then to Chetumal, where you can get a very nice bus (ADO) to Cancun. First class Mexican buses are comfortable, with reserved seats, videos, etc. Taxis from Corozal are expensive -- ones on the Chetumal side are fairly inexpensive. Taxis do meet the incoming flights at the Corozal air strip.
You might also want to think about spending a night in Corozal Town. Delightful little place on the bay, friendly, safe, several good hotels and restaurants, much cheaper than most other places in Belize.
--Lan Sluder
Belize First Magazine
http://www.belizefirst.com/
AmbergrisCaye.com 
