The Cancun International Airport has a FREE shuttle service between terminals (terminal to terminal), very easy to use. It works for those passengers that must go from one terminal to another, if that's required by their travel itinerary. The airport shuttle bus between terminals is secure, comfortable and runs punctually every 10 minutes.
You must note that in the Cancun Airport there are three passenger terminals: the International flights board and disembark in a different building than the charter flights or the domestic arrivals and departures. Take a look to tour travel documents to be sure in which terminal you arrive and must be picked-up by your ground transportation supplier.