You are right on KYB !
The Orvieto Hertz office is the most convenient and easier than Chiusi.

Thanks to all! Orvieto might be a good alternative, although I was planning on a stop at Bagnoregio first for breakfast... does the train stop there?... and then on to Civita-Orvieto-Cetona-Montepulciano-Pienza , on my way (2 day one night drive) to San Gimignano...

I assume not---but the backtrack is not that far after you get the car.

Just to throw another log on the fire, the car rental offices at the airport are open all hours. You could pick up your rental car there.