I would disagree that renting a car is similar to buying a plane ticket. You rent a car for a period of time and the longer that period the more it costs the car rental company and the more you expect to pay. A plane ticket is not period dependent i.e. it doesnt cost the airline any more if you buy a ticket to travel in a months time or in a month and 1 days time.

I could understand that if I wanted to increase the rental period, I could not expect to get an extra day for the same price as a day in the original booking. but to not allow me to reduce the rental perod without penalising me (I was willing to pay the same price) is ridiculous.

At the same time of changing the car rental contract, I contacted a hotel in Jackson and asked if my booking could be modified in the same way (arrival a day later). "No problem" was the response.

The hotel is obviously happy to get a room back to rent (it is sold out now) at a much higher room rate then they would have charged me