Rental Cars in Mexico

A couple of years a go we rented a car from Hertz at the airport. They would not us have the car without their insurance. There were 3 of us and we were driving to Patzquaro to tour the area. When we returned the car, the Rep stuck his hand under the engine and had oil on his fingers. We were told we had damaged under the engine and had to pay now. We were using our AMEX card and turned the problem over to them. Hertz did not respond to AMEX and a period of time later our credit card was reimbursed for the repair bill.