Resort Rent-a-car: we had to fix their errors

My wife and I rented an Explorer from Resort Rent-a-car in October. We had the vehicle three weeks, one-way from Denver to Phoenix.
While we had the Explorer, several minor problems occurred with the car. Annoying but no big deal.
A weeks after returning the vehicle, we were called and asked where it was - why we hadn't returned it. We spent about a month trying to convince them we didn't have it. This required several long distance calls and several hours time. We also received a registered mail notice that we would have to go to the post office to claim.
Then, we got our credit card statement and they had charged us TWICE! Back on the phone three or four more times to straighten that out. We finally talked to the company president who was not very understanding of all the time and effort we had taken to fix their errors. We asked for a significant portion of our rental fee to be returned. He denied, offering us a free weekend rental in the future. This was no use to us, as we have no plans to visit the area in the near future. Plus, why would we risk the hassle with them again?