A car company that I will recommend is AutoEurope. They have prepaid rates which guarantees the rental rate, insurance and all the little extras that they don't tell you until you get your creditcard statement at the end of your trip.

I've used them regularly and despite the prepaid fear.. ie no upgrades etc, their rates have always turned out lower than most agencies and I have gotten upgrades half of the time.

This is their website: http://www.autoeurope.com