in the fwiw category, I called back Budget a few minutes ago, and after grilling the customer representative who answered, he reiterated what the other customer rep said...

"you have 15 hours from the date and time of your reservation pickup, to pick up your car--your quoted rate is good during that period. If you do not pick up your car after 15 hrs, your reservation will be cancelled."

sure not what I expected, but works for me, as I have a reservation for tomorrow and I anticipate I will be several hours late in picking up the vehicle.