You used to be able to fly from Cancun to Belize City on Aerocaribe, but that service has been canceled.
Check out this web page http://www.cybercayecaulker.com/Gettinghere.html
Here's an excerpt from that page:
We've done the Cancun/Belize bus trip a few times. In our area, we can get some incredible deals to Cancun, as low as $176 RT from Detroit on the charter company (Northwest MLT). After arriving in Cancun and clearing customs and immigration, go to the shuttle bus/van ticket counter and purchase a ticket to the Bus Terminal (there is only one bus terminal). The busses to Chetumal leave Cancun about every hour, make sure you take the First Class (Premiere Classe) or Express bus, this leg of the trip takes 5-1/2 hours. However, if you get to Chetumal after 6-6:30 p.m., you'll have to spend the night, or go to the Nuevo Mercado (New Market) bus station and take a Venus bus (which is similar to the old style Bluebird School Busses). WHen crossing the border, ask the bus attendant to help you get a double entry stamp on your tourist card (costs 50 pesos) if you do not receve this you will have to give up your tourist card at the border and repay the $25 u.s. exit tax for Mexico again when you return to Cancun. (this tax is paid at any Mexican Bank, you pay the tax and they stamp your new tourist card.) I believe the earliest first class bus with Batty Brothers Bus Line into Belize leaves around 10 a.m. In Chetumal we stayed at the Hotel Ucum, for $10 plus a $3 cab ride. It was very clean and quiet, right around the corner from the more expensive Holiday Inn, but one block from the Mayan Cultural Center. When you return to Cancun via bus, remember to hold on to some pesos, because the bus companies will not accept U.S. or Belizean currency, and we had the misfortune of being there when the exchange was closed. We got the convenience store to exchange money for us, but at a much lower exchange rate, kind of a racket going there. If you get to Chetumal too late for the Beize bus, you can hire a cab to take you to Corozal, just over the border. If your ultimate destination is Caye Caulker, don't forget that you can fly from Corozal, and you don't have to go all the way down to Belize City.