Car rental in Tuscany

www.Autoeurope.com and www.kemwel.com are two consolidators part of the same corporate umbrella group. I've used both with no complaints. They don't rent cars directly, they are consolidators, i.e. intermediaries between you and a rental company. They usually get you a rental car at a cheaper rate compared to going directly to the rental company. They deal primarily with the following rental car companies in Italy:
Avis-Budget, Europcar, Hertz, Maggiore.
I recommend you check their prices in both websites (as prices might differ, although they are part of the same company).
Europcar does not have an office in Orvieto, therefore if you use Autoeurope or Kemwel, and you select an Orvieto office, then the rental with be with one of the other 3 (Avis, Hertz or Maggiore).

Auto Europe, as already suggested, is our preference too, or also Europe by Car. Find out about these consolidators and more about booking a rental car at http://www.ricksteves.com/travel-tips/transportation/booking-a-car.