"Should" and "legally must" are not necessarily the same. Have you called Thrifty rather than just checking the Web site and posting here? Rental companies can't necessarily guarantee the specific type of car because they don't always know in advance who will rent what, and who will return what, before you show up. Would you accept a larger van if the one you reserved isn't available? Did you pay in advance? An online reservation doesn't count as a legal contract as far as I know. Thrifty can't charge you and then not provide the product or service, but so far that's not even a question. You have just made a reservation, and most rental sites say things like "Chrysler 300 or similar" and/or that they can't guarantee a particualar type of vehicle will be available. If you phone they should be able to tell you if they usually have a large number of vehicles like you reserved (or better). This can vary by location (e.g., airport locations typically have more vehicles). Thrifty may well help you get a vehicle at another company's location if they have nothing, but that doesn't mean they have a legal obligationt od o so or that they would guarantee the rate. You have the ultimate power to rent somewhere else if you don't like what's available when you arrive (I've done that).

People can back out of a rental for no reason (even without notification). I'd phone Thrifty with any concerns. Sure, be insistent, but unless you are in their frequent renter program, your influence may be limited.