Excellent and inexpensive bus service runs along the Tulum corridor (main highway).  Taxi service is available from the corridor to the hotel, to the airport, and to many other points of interest in the area.  The cost of a  rental car should be about $140 per car for the entire week.  Reservations may be made in advance with any major rental agency in the U.S. such as Hertz, Budget, National, Dollar, etc.