As I've been researching car rentals for my upcoming trip, I've been finding the best rates through www.autoeurope.com. It's a US-based broker that books through several companies such as Hertz, Avis, and Europcar. But you can sometimes know exactly which company you'll be getting based on the address at which you'll be dropping off or picking up.

Minimum rental is for 3 days. Prices are quoted in USD. Rental price is prepaid, but they are cancelable/refundable until just a few days before arrival, IIRC. Picking up at an airport may result in additional cost, but dropping off at an airport does not. There are typically no surcharges for one-way rentals within the same country.

For my 3-day rental in Tuscany, I'm looking at booking a 4-door compact for about $217 total. For 4 adults with luggage, you'll need bigger than that.