The usual drop off point in Venice is Pizzale Roma. It is a huge garage that you essentially dead end into when you arrive at Venice. Couldn't be easier to get to and to return a car there. Then you can literally step onto a boat from there to get to your hotel. It's SO much easier and more time saving that driving to the airport and then getting back into the city.

I would think you could still change the drop off point on your contact -- it's sure worth a try.

Unless you were planning to make stops along the way from the CT to Venice (probably a 4+ hour drive without any stops), you could drop the car in La Spezia as you come from Florence and then train to Venice when you leave the CT. If you drive, make sure you know the hours the Sixt office is open/closed that day.

been using Hertz last few years,great service. they have even added some automatics to rental fleet. before that eurocar. had some billing issues with eurocar.i always get my intn'l driver lc./ they will ask you for both, but if you get pulled over or road checked by polizia, give them the internation. also bring a photo copy of your US driver lc.