Dollar Rent A Car review: Deceptive practices

I have been a long-time customer of Dollar and Thrifty rental cars. I arrived this past weekend to Boston Logan Airport for a 28-hour rental. When I booked the trip, I also booked the rental car. This was months prior to my trip. When I finally got the counter, I was told that the rental car class I booked was unavailable. In order to get a car, I had to upgrade the class of car. This of course came with a charge. I told the counter representative that I did not agree with this deception and was not interested in paying extra when I already booked a car months prior. She told me I had to pay the upgrade fee or I did not have a car. I was on a time-frame in getting to my destination, therefore, I paid the charge and decided to dispute it later. Upon returning my car in pristine condition and filled up with gas, I reached out to customer service and told them of the concern. Their response: "you signed the rental agreement at the counter, so the charge will stay." Absolute deception and blasphemy. Not a good way to treat your customers. Will not be using Thrifty or Dollar again.