I don't know if it's the absolute closest location, but 53-55r via Borgognissanti is 2-3 blocks from the Florence train station. Auto Europe arranges rentals through Hertz, Avis, and Europcar; if you book through their site they'll give you an error message if you ask to return the car to a different company than you picked up from in Torino. (To tell which company has which location, you would need to go to the company's Web site.) Any of the Borgognissanti locations is pretty close to the station. You could also try different combinations of locations (using the different companies) to be sure the prices are comparable; the Europcar locations at least are offering free upgrades.

Andrew Missouri