Good car, good service, untold charges

Thrifty didn’t have a hybrid car when I came for pick up. They offered a full size car. I don’t want to wait any longer due to long flight plus time zone difference. The receipt shows a few dollars more than the reservation. I am okay with it. The car is in good condition and very clean. However, when I returned the car, I got a receipt that charged double the reservation amount. The per day rate went up to $45 instead of $30 at reservation. I was in rush catching up the flight, that had no time to argue with Thrift. Very annoyed.