I usually rent through Autoeurope before I go - the rental is through a major agency, last time was Avis, but I can prepay it at booking and nail down the rate in USD. The Autoeurope rate will include almost all of the mandatory insurance, taxes and fees; the only add-ons I end up with are the road tax and extra driver charge. Keep in mind theft protection insurance is mandatory and car rental agencies on Italy will generally not accept insurance through a credit card or personal insurance. And even if you take the CDW most will charge excess charges if there is damage to the car.