Not to spoil the pot, but have you tried AutoEurope prepaid service. Their car rates for Europe for rentals longer than 3 days are second to none and chances are you will see that their prices (and convenience) will beat most car rental agencies in Europe.

http://www.autoeurope.com/

Having recently rented a car in Germany with Avis (which was the cheapest I might add), even for one day, I was shocked at the 160 euro final bill for an automatic car!