Surely you jest, when it comes to applying an 18:1 exchange rate to the USD-MXN exchange rate. That represents nearly a 50% overcharge, and I´ve never experienced anything like this in dealing with any company in Mexico, including my experience in renting cars on two different occasions.
I am saying that if the system allows people to cheat, they will. And that is happening in Cancun and other parts of Mexico since the system is corrupt. Once this corruption stops this type of complaint will disappear. Again this is not right, but it is the way it is due to the corrupt system and people being able to get away with such theft. And if the OP would have objected and the police was called, who knows this may not be even posted here. Although with a corrupt police force it could have turned into a 25:1 exhange rate. Who knows. But as I said this is the way it is and the OP regrettably will not get any further satisfaction from Dollar in the US since the Cancun office is a franchise.
I had problem with my hotel once and someone refered me to the consumer protection office. Luckily, their office was just 15 minutes walk from my hotel who, by the way wanted to charge me 3 time more what was on the original bill because it was high season. I went to this office to ask a single question. The girl took my bill, call the hotel and in two minutes the problem was solved. As I understood, they had to respect what was agreed or this office will sue the hotel. WOW. ProcuradurÃa Federal del Consumidor