ive booked rentals cars on priceline before. that only works with airport locations.
you put in your airport, the dates you want the car, and the car class (compact, full size sedan, small svu, etc), and your daily bid amount. it works the same way. if they accept it you are charged on the spot and cannot cancel, if not you can try another car class or wait a day.
i got a mazda 3 at the tampa airport for $14 for 8 days this way. also got a cadillac xlr for $48 a day in tampa. the cheapest i did in boston was a ford fusion for $90 or something like that. boston's car rental taxes at the airport were insane.