@ying.wang
I did not send an email.
You can secure drop-off directions at any time. But, yes, on the day before or the day of, you should call the rental office and give them your car details, since you did not get the car from their location. The most important reason to do this: if you get a ticket and your case goes before a judge, the judge will ask you if you notified the car rental office of the date and time of your drop-off, the make/model of your car, and its license number. I promise you, the judge will not ask you if you got instructions from someone on the internet. If you dot all your i's and cross all your t's beforehand, you'll better protect yourself down the road.
FYI, Garage Europa claims on their website that if you follow their directions and you still encounter a ZTL camera, they will help you in reporting that. I wouldn't worry so much. It may be annoying, but this is not rocket science.
Garage Europa has a decent website with an English option. I don't know how up-to-date it is. They also offer a ZTL link page:
http://www.garageeuropafirenze.it/en/no-ztl-parking-center-Florence.php
On that page, you can find a Download Map with two options to reach them without incurring a ZTL fine. The blue arrow route is the route that I've taken for many years while conducting business in Florence. I just drove this route this past October:
FROM THE SOUTH:
Exit Firenze Impruneta. Drive north on Via Senese (which originates as the famous Via Cassia SR2). Turn right on Via del Gelsomino. Turn left on Viale del Poggio Imperiale, and drive north to Piazzale Porta Romana. Enter the large roundabout and turn right at the street, Viale Francesco Petrarca with the city wall on your right. At Piazza Tasso, Via Petrarca turns into Viale Vasco Pratolini. Turn right on Viale Ludovico Ariosto. Turn right on Borgo S. Frediano. Turn left on Via Sant'Ofrio. Cross Ponte Amerigo Vespucci and continue straight onto Via di Meleganano. Turn left on Borgo Ognissanti, and in about 300 ft, Garage Europa will be on your right. You can't miss it. It's an ochre-colored building with a large GARAGE sign fixed perpendicular to the facade. The Avis/Budget Car Rental office is on your right, street level, just before the huge Garage Europa entrance.
By calling your Borgo Ognissanti office, you can confirm these directions for the day and time of your drop-off. You can also call the Garage Europa for confirmation, if necessary. Several guys there speak English.
This route is easily viewable using Google Street View. With the Google Man, you can drive the route and practice it as if you're driving your own car. Once you drive this route and discover how easy it is, you'll wonder what all the fuss is about.