Transportation in Cancun can be a bit of a challenge. When you get out of customs, you will be accosted by aggressive car rental sales people and once you step out of the airport doors there are even more aggressive taxi agents.
Last time I was there (in June) we checked the rental places at the airport and they all had horror stories on Yelp (https://www.yelp.com/search?cflt=carrental&find_loc=Canc%C3%BAn%2C+Quintana+Roo%2C+Mexico ) and other review sites. Insurance requirements in Mexico are complicated and most rental agencies (even big brand names) are trying to take advantage of their customers (to put it politely). Example: my credit card specifically offers CDW coverage and I have a written proof of this. I called one one brand name agency and submitted this document as they asked and the still would not confirm this. Their answer was "rent first, go to the local office and then the rental agent will decide whether they accept it or not", which is in my opinion an utterly unacceptable business practice.
We ended up renting with a local outfit called Easyway (no advertising intended) and were overall happy with the experience. They were upfront with the prices and didn't try to upsell or add-on more mystery fees.