AutoEurope or its sister company, Kemwel, are American brokers situated in Maine that shop around and reserve a car for you with a local agency. The main advantage of renting through AuroEurope or Kemwel is service. Should there arise a need for any reason, you will have an American company to work with and people who speak English.
I never even bother shopping around but you certainly can and AutoEurope will match lower price estimate, should you find one. You can make as many changes to your reservation as you need, and you can cancel anytime prior to the time of your reserved pick-up without charge. This may actually work to your advantage here while you are finalizing/rethinking your plan.
We picked up a rental car on Borgo Ognissanti and headed straight out of town without an incident.
Edited: 2:13 am, January 18, 2013