Does anyone have any experience adding an authorized driver for just one day? The only question on point seemed to be this one, which never got any responses, so if anyone has ever added or tried to add a driver to rental for a single day, especially on the very first day of the rental only, please let me know.

Did that with Avis in Germany. Visit location, take additional driver off the contract. No problem.

I once rented a car in NY from Hertz and drove to Montreal. Because it was a US-based origination, they had to use some sort of manual slip to add the additional driver. Because it never went through the computer system there was no charge. Probably wouldn't work for the same country, but I just found it an interesting nuance of Hertz's system. If you wanted to try, you could rent initially without the additional driver and then stop at another location to have them add them to see if it "falls through the cracks".

As bhomburg pointed out, you should be able to remove the additional driver after the fact, but don't necessarily count on the counter agent being able to figure that process out. It's not something they do every day, and of course YMMV depending on company.

All of that said, if you are renting in the US, you might consider renting using the Costco, AARP, AAA, USAA discount codes as all of them alloow for an additional driver.

When you say "visit location," do you think a different location of the same agency would work for getting the additional driver removed (notwithstanding AutoSlash's YMMV concerns)? This is a non-spouse additional driver for a foreign rental: Costco rates haven't been competitive, and the USAA code seems to get me a spouse or traditional USAA member as an additional driver.

Yes, a different location should theoretically work at least with the major firms, but keep in mind that depending on where you are renting, certain locations may be franchises or licensees as opposed to corporate locations, so that could have an impact.

Also note that I said earlier, "if you are renting in the US, you might consider renting using the Costco, AARP, AAA, USAA...". Outside the US, the additional driver benefit usually does not apply. I know you said you already checked those, but just a point of clarification for others that may stumble over this thread later.