Originally Posted by jackal

The rental computer systems (as opposed to the reservation systems) are generally not sophisticated enough to modify the rate once you've booked it. The exception would be if you're booking a weekend rate that has certain time-based criteria (minimum length-of-rental, time/day restrictions, etc.), in which case, yes, you would fall back to the more expensive rate. If this isn't a weekend rental, though, I can't think of a time I've ever seen anyone's rate change because they picked up early.