I just got back froma quick trip to Cancun, and found out that these guys are total crooks.
I made my reservation online, and am mid-tier with Hertz, so I'm pretty used to how things go. When I arrived at the location, the lady at the Gold Plus Rewards desk started treating me as if it was a whole new reservation, then she added the top-tier insurance package without asking, then told me they didn't have any full-size vehicles available but I could pay more for an SUV, and so on. IT was a nasty, nasty experience, but fortunately I speak decent Spanish and pushed back at all of this until my VW Jetta magically appeared and they agreed to leave my reservation as I had made it online.
Oh, and then when I was about ready to head outside and get my car, she mumbled about being charged an extra 5% fee if I paid with my Amex card.
But the real crime was afterwards. Turned out, the gas gauge in the car did not work. It showed full when I left, but later having dfriven for a few days, I went to fill up the car, and afterwards the gas gauge only showed 3/4 full. I throught the Pemex station had ripped me off, so I went to another one and asked them to fill me up, but the attendant said the car was already completely full, yet the gas gauge showed only 3/4. At which point the attendant pointed out that this is a common scam done by some rental car companies there.
Sure enough, arrived to return the car, and they said it wasn't full. It was only 3/4s so they would charge me for the extra gas plus a fee. I had to argue with them until they physically saw that it was impossible to add any more gas, and it was indeed full. Finally they relented.
In the end, I escaped having paid only what was on my original online reservation. But it was a nasty fight all the way to keep from getting ripped off, and I'm sure I was only able to come out of it the way I did because I was calm, unyielding, and spoke Spanish.
Also: every single person asked me for a tip. From the guy who drove me from the terminal to the rental car, to the lady who tried to scam me at the GoldPlus counter, to the guy who brought me the car, etc. Every single one without any shame asked me for a gratuity, even after trying to rip me off.
I've been going down to Mexico very regularly for work (usually Mexico City) for the last 4 years or so, and I've never had a bad experience (and that includes getting hit by a car and ending up in the hospital.) I love Mexico and the Mexican people, but this experience in Cancun, especially with Hertz... I've never felt so much like prey surrounded by cold-blooded predators as I did during this trip. And the top predators were the people at the airport Hertz location.