First of all a welcome to FlyerTalk for the OP!
While I have to agree that I despise PT Cruisers, and the OP's situation would be enough for me to go to another agency or pay to upgrade, there was no bait and switch involved unfortunately. Thrifty classifies the PT as a midsize, which is consistent with every other agency IME. Midsize is a catch-all category these days, housing everything from the Kia Spectra to the Mazda 6, and everything in between. I've seen the PT as a midsize at Budget, Avis, National, and Alamo for certain, along with its competitors the Rondo, Matrix, and HHR. Another reason for the unfortunate PT-ville experience of the OP is that Dollar and Thrifty are predominantly fleeted by Chrysler, and when people don't want to buy PTs (for good reason), guess where they go?
