ripoff and horrible customer service

I rented a car for a week, then ended up extending for another week and three days. I paid for the first week up front at a certain rate. I rcvd a very nice car, but I asked for the compact/economy class and this car was a class higher at a rate higher. When I called to extend I was told it would be the exact same weekly rate so I was happy. When I brought the car back, I was shocked at my bill. They charged me an additional $20/week, even for the first week I already paid for. When I tried to speak to manager I was told nobody was available, but when I went into the office I saw them. The rep argued with me and basically stated they can change the rates whenever they want and that I can just dipute it with my bank. What happened to customer service? The low rates lured me in and they turned out being a total ripoff.