I've had this happen a few times. I'll look up rates and then log in to make the reservation and the rate goes up. In my case, it's usually that the fullsize is actually cheaper than the midsize but as an excutive they give me the midsize rate. Unfortunately, it makes the rental more expensive. In the end I just rent the fullsize w/o logging in and grab something off the aisle and show my card at the booth. I've yet to have a problem.