This is how I have done the trip from Cancún airport to Belize…
From Cancún airport, ADO Riviera airport shuttle buses depart approximately hourly to Playa del Carmen. Tickets are sold at an ADO counter in the airport just after exiting Customs, or from the ticket seller where the bus is parked. To get to the bus, go out of the airport terminal, turn right and keep going to the other end of the terminal. The bus trip is about 1 hour and costs MX$120 (as at January 2013).
The airport shuttle buses terminate in Playa del Carmen at the bus terminal at the corner of Quinta Avenida and Avenida Juarez (5th Avenue and Juarez Avenue). If continuing to Chetumal, get a taxi or walk to the other bus terminal (Terminal Alterna) at Avenida 20 and Calle 12. Sometimes the airport shuttle bus stops at Terminal Alterna on the way to the 5th Av. terminal.
Here’s a link to a map of Playa del Carmen showing the location of the two bus terminals:
From Terminal Alterna in Playa del Carmen, ADO express buses depart every couple of hours to Chetumal. The trip takes about 4.5 hours and costs MX$252 (as at January 2013). The bus might make stops at Tulum and Felipe Carrillo Puerto before terminating at the ADO bus terminal in Chetumal.
The options for continuing to Belize from Chetumal are Belizean bus, taxi and water taxi — see the next section, TRANSPORTATION BETWEEN BELIZE AND MEXICO, for details.
Other options from Cancún airport to Belize or Chetumal
Instead of getting a bus to Playa del Carmen, some travellers prefer to get a bus from the airport to the bus terminal in Cancún City where they can get an express bus to Tulum, Chetumal or Belize City. Cancún airport is located about 18 km south of Cancún City and about 50 km north of Playa del Carmen, so this means first travelling north to Cancún, then getting a bus back south, but it avoids the bus terminal change in Playa. A popular choice is the daily overnight express bus from Cancún bus terminal to Belize City.
A transfer service or private shuttle is another option. Here are links to some services:
a) Belize VIP Transfer Service, http://www.belizetransfers.com
b) Moralez Travel Service, http://www.gettransfers.com
c) Cancún Shuttle: http://www.cancunshuttle.com/
d) Cancún Express: http://www.cancun-express.com/
e) Cancún Airport Transfers: http://www.tucankin.com/
Tropic Air will begin flights between Cancún airport and Belize City, but has not announced a date yet.
This is how I do the trip from Belize to Cancún airport…
I get a bus to Chetumal, but there are other ways (taxi, transfer service, water taxi). From the Chetumal ADO terminal, express buses depart every couple of hours to Playa del Carmen. The trip takes about 4.5 hours and costs MX$252 (as at January 2013).
The bus arrives in Playa del Carmen at the bus terminal at Avenida 20 and Calle 12 (Terminal Alterna). If continuing to Cancún airport, get a taxi or walk to the other bus terminal at Quinta Avenida and Avenida Juarez and get the ADO Riviera airport shuttle bus to Cancún airport. The bus trip takes 1 hour and costs MX$120 (as at January 2013).
Other options for getting from Belize or Chetumal to Cancún airport
A popular choice is the daily overnight express bus from Belize City to Cancún City because this bus stops at the airport (see next section for details).
Another option is the ADO airport express bus from Chetumal. Buses depart daily from the ADO Chetumal bus terminal at 00:30. The trip takes 5.5 hours and costs MX$342 (as at January 2013).
Other options are transfer service and Tropic Air flight (whenever they commence flights).
Always check the ADO or ticketbus website for up to date schedules and fares. The ADO site is in Spanish, but the ticketbus site has an English language option.
Note 1: MX$ is Mexican pesos. On the Mexican bus websites, just the dollar sign is used to represent pesos.
Note 2: The ADO express buses have very good air-conditioning; dress warmly.