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My understanding is that Third Party Liability is mandatory in Mexico.
I have rented from from National in Cancun, and it was fine.
We paid for the TPL, and were charged what we were quoted.
I’ve also heard good things about a couple of the independent places, but don’t have any experience.
It’s not what you asked, but I’d also agree with some of the comments about whether you need a car in Cancun.
If you’re going to the typical sites, a car may be more hassle than it’s worth!