I've used Sixt and Europcar in Italy. How about Autoeurope or others?
I've used AutoEurope several times. They are a consolidator, so you don't know what car company it is until you make your selection. No problem if you want to make changes or cancel. You could compare all three companies and then choose what works for you.
You will have trouble finding a better deal than you can get at gemut.com.
http://gemut.com/rent-a-car-in-europe.html