Furthermore, at a rental car center, you're not just swinging in and swinging back out. It could change of course but they're setup that once you drive in, you're returning the car. (Re: comments about rental car center being the charger of last resort.)

(I had a friend joining me on a trip a number of years back. We wanted to add them to the contract once they arrived. I had to patiently explain that we could do it with a company that also had an off-airport location but we couldn't drop by the car rental center at the airport. I've had other experiences with how car rental companies really don't have good mechanisms to change things once you've signed and initialed the contract.)