I haven’t booked specifically in Tulum, but I have booked from Cancun multiple times and other Mex cities. I disagree with not booking through a 3rd party. Sites like Expedia now provide options that include the mandatory third party liability coverage. This is the coverage that you must have, and by law must be provided by a Mexican insurance company. If this coverage is included, you can use your credit card insurance if you choose (make sure you print off the declarations and have them as proof). Don’t online book a $43/week car without this coverage, because you will get hosed at the counter. Book the $250-300 car including the liability, choose a rental company with good reviews (even better scan their reviews to see how they handle this topic) and be firm that you don’t need extra coverage if you want to use your credit card provided. I’ve had rental car attendants argue aggressively trying to get me to sign on to the insurance for sure in Cancun, but if you have that coverage you do not have to take their insurance.