Thanks for the info! So, online at the National website, my default car size comes up as a Midsize. Do I book a midsize and then pick up a Full size (or a convertible or premium if I get lucky) or do I book a full size and then at checkout the price gets lowered to Mid size?

I usually Priceline my car rentals but if I have a shot at a convertible, combined with the rent 2 get 1 free, I may just go direct this time