I'm just putting in a plug for Get Transfers, the company out of Corozal run by George and Esther Morales. We used their service on Friday to get to Merida, and they could not have been nicer or more accommodating. They met us at the airport on time in their huge 15-passenger van, and we easily fit all of our luggage, boxes, pets, and ourselves into the van. The ride was very comfortable, and George made getting through the border a seamless process for all of us.
If you have somewhere you need to go in the Yucatan or in Belize, I cannot recommend this company highly enough. It is well worth the money.