I got the link in my carfirmation today. Showed a Crown Victoria for $17/day extra and Cadillac for $44/day extra. So I guess this is just for paid upgrades.
I got the added bonus this morning of arriving at the check-out gate and being told, "this car isn't in the system" which was interesting since it was the exact car, license plate and all, that was in my carfirmation. So I had to go back and take a gold choice.