Jimmy Geldburg:
What happens if you're late picking up a rental car, for which you
have an unpaid reservation with a very low guaranteed rate?

Let's say I have a car reserved at noon on a Friday, for two days. If
I pick up the car at 4 pm Friday, would they:
1. Honor the rate, and set the return time to 4 pm Sunday
2. Honor the rate, but I'd have to bring it back at noon Sunday
or
3. Make me pay the much higher walk-up rate

There's also

4. Have no cars available because they already assumed you weren't
coming and rented the last one to someone else.

I have no idea at what point that's likely to happen, but whatever their
policy is on the rate, you should at least telephone them -- preferably,
at the specific location where you're picking it up -- as soon as you know
you're going to be hours late.
--
Mark Brader "We can get ideas even from a clever man." ...
Toronto "Yes, I think you can. Even ideas you should
have had yourselves." -- John Dickson Carr

My text in this article is in the public domain.