Capital Rental Car- U-save

Correct me if I misunderstood. The car rental had a charge of $xx USD which they converted to pesos at a rate that is very favourable to them and then the rental company had the customer sign a charge slip for the amount that the rental company computed. I can’t see how the card company can help when the amount on the slip is for the total that the customer agreed to.
I’m not blaming the customer in any way and I hope I’m wrong but it just seems like another typical Mexican rip off. A family member was discussing how, after spending three months in Puerto Escondido, the most important thing he learned was to make sure to count his change every time he was handed change. “ oh, sorry, my mistake”. It is amazing that a country with so much to offer is so corrupt.