corozal.com has this to say about rental cars in Chetumal (guess Avis is now Aventura Maya):
Continental Rent-A-Car has their rental desk in the lobby of the Holiday Inn. Rates range from about 200 pesos per day for a VW sedan, to about 460 pesos per day for a Tracker. Mileage is about 1.25 to 1.50 pesos per kilometer. There are weekly rates also. FAX: (983) 2-24-11. TEL: (983) 2-11-00 ext. 191.
Aventura Maya has a rental desk in the lobby of the Hotel Las Cocas. FAX: (91 983) 2-05-44. TEL: 2-09-20. You can also get a message to Aventura Maya by addressing it to the hotel's email address: [email protected]