Hertz drop off location in Florence

Florence has a unique way of numbering addresses. Most Florentines don't even know.
Street numbers are based on the flow of the water of the River Arno. The river flows from East to west and splits the city center roughly in half. The souther side (actually less than half) is called Oltrarno. For streets which are parallel to the river, street numbers increase in the same direction as the river flow (I.e, from east to west). For Streets that are perpendicular to the river (north-south direction) numbers increase as you move away from the river. Even numbers are on the right, odd numbers on the left.
But there is another twist. Shops have red numbers (137/r stands for 137 red for the Hertz shop location) while other addresses (like residences) have black/blue numbers. The two numeration so are not related and in fact the Garage Europa, at 96 Borgo Ognissanti, is exactly across the street of the Hertz office/shop at 137/red.

Thanks Roberto. I was hoping to find someone who would actually dropped off a car at this location. I think 96 might be where you pick the car up and 120 7R might be the drop off location but so far there's no way to find out.

DIRECTIONS
I can't give you precise directions if I don't know where you will be coming from.
The Garage Europa is inside the ZTL (Limited Traffic Zone). To avoid getting fined by the camera follow these instructions. Go to Google Maps directions.
Enter:
From PIAZZALE DI PORTA ROMANA
To: BORGO OGNISSANTI, 96
The site should route you through viale Petrarca and eventually across the river on Ponte Vespucci.
Follow that exact route. You miss one turn, and you will be paying through the nose. The City of Florence has no mercy.

Roberto, I just read your reply again. That should teach me not to read through something too quickly. That's great that the 2 addresses are across the street from each other (didn't look like it on the maps) with this information we should actually be able to find the drop off location! Grazie, grazie mille.

As I mentioned, the Hertz office is across the street from the garage. There is no way to park the car inside the shop. That is where you do the paperwork, but cars are inside the multistory Garage Europa across the street. If you oick up there, first you do the paper work in the office at 137/red Borgo Ognissanti, then you cross the street to the Garage Europa at 96 Borgo Ognissanti, get on your car and drive away. When you exit the garage you will turn right.
When you return the car you do the reverse. First you enter it in the same garage with the garage. Then with the papers filled by the attendant, you walk across the street to the shop, and settle the account.
If this process has stressed you ( I'm sure it has), next to the Hertz, at the corner with via Curtatone, there is a wonderful large coffee bar which sells great pastries (I think it's still called Bar Curtatone). In my youth, the Business School of the University of Florence, where I went to school, was located on that same street, so that bar is where I gained several pounds between a class and the next (and especially before final exams). I'm sure you can release some stress with some pastries and an espresso or cappuccino. They also serve drinks of course. If you get in trouble for excessive drinking, I'm sure the US Consulate General (also located less than 300ft away), will be able to assist you.

There is an alternative to dropping the car in town - drop it at the airport location . It's easy and you don't risk the stress or the large fine for dipping into the ZTL . A short bus ride from the airport terminal takes you right into the center of town

The Business School has left the place that is now a fashion school.

While Borgo Ognissanti 96 is nominally within the ZTL, the section of Borgo Ognissanti between via Melegnano and Il Prato can be driven freely. IMHO the easier and safer itinerary is:

Piazzale di Porta Romana - viale Petrarca - viale Aleardi - via del Ponte Sospeso - Piazza Taddeo Gaddi - cross the Arno river at Ponte della Vittoria - as soon as you cross the bridge keep right and and enter Piazzo Vittorio Veneto - do not enter the Lungarno (you get a fine but you go nowhere) but turn left, coasting along a large parking area on your left. At the old city door (Porta a Prato) turn right (il Prato) - Via Curtatone - Lungarno Vespucci - turn left at the Vespucci bridge entering via Melegnano - left to Borgo Ognissanti 96

Now I remember why we didn't rent a car in Florence. A vote for the airport.

Picking up at the airport will cause you to incur additional rental fees. If you just return it at the airport (not pick up), then no fees will be incurred.
Therefore pick up your car on Borgo Ognissanti (very easy to get out), but if you are afraid of the ZTL return it at the airport or at an out of center location.
See my comment on Andrea's post on Florence pick up locations.

Thanks, everyone. We are picking up the car in Rome and returning it in Florence. Unfortunately, we cannot return this car to the airport. This location is our only option. We will 1st drop off our bags at our B&B, Casa di Annusca, located at Via dei Serragli, 126 in the Olltrarno, somewhat near Santo Spirito. We were able to get a street view of the Hertz office and car return through Google maps. So far, so good. We also got directions from Google maps which differed a bit from the directions offered by via Michelin. And yes, we are very interested in avoiding fines. Been there, done that. Didn't like it. Our B&B host said he could let the powers that be know our license plate number because otherwise there would be fines. I assumed this was just about getting to the B&B but perhaps he was referring to getting to the car return. Any further thoughts? Again, grazie mille.
PS: Roberto, it must've been great to go to school in Florence. We will certainly take advantage of the stress relieving properties of Bar Curtatone although it might be a hard choice between the pastries and a glass of wine.

The B&B can have your license plate placed on the City website (the White List), but only for the B&B.

To get to the B&B, you still need to go to Piazzale di Porta Romana. Coming from Rome the A1 exit is FIRENZE IMPRUNETA.

Once you reach Piazzale di Porta Romana, you drive on Viale Petrarca along the City Walls until Piazza Tasso (first opening on the walls). Turn right on Piazza Tasso. You will go under the ZTL camera at that juncture. As soon as you pass under the camera turn right on Via del Campuccio. Go straight until you reach via de Serragli. Turn right (it's one way, so no choice there). The no. 126 will be on your right. Via de Serragli is very narrow (one lane) but you can park on the left side (if there is room). If not park in front of a garage or something (one will stay in the car while the other drops off the luggage).
When you drive away, you have no choice but continue on via de Serragli until it puts you again on Piazzale di Porta Romana. From there you follow the directions I gave you above on how to reach the Hertz office on Borgo Ognissanti.

Since you are coming from Rome, you could save yourself a lot of hassle if you would return the car at Hertz on 53 via del Sansovino, far from any ZTL, then take a taxi to the B&B from there. There is no extra drop off fee for returning it there instead of Borgo Ognissanti.

To reach the Hertz on Via del Sansovino, exit the A1 freeway at the FIRENZE SCANDICCI exit. Then follow the directions to the city center (CENTRO, often symbolized by a target symbol). It's all straight from the freeway exit along viale Etruria, which becomes Via del Sansovino. Hertz will be on your right.

By doing so you don't run the risk of getting lost and in the end you will save time. The taxi fare from via del Sansovino to Via de' Serragli is probably under 15€ (it's just a mile or so away), so not really a huge amount. For sure less than a fine if you happen to take the wrong turn.

Thanks, Roberto. The location you suggested is, indeed much better. Unfortunately, we will be returning the car after their closing time on Saturday and, although we have the car until 11 AM on Sunday they are closed on Sunday. I'm thinking that it might be worthwhile to try to return the car on Saturday before their closing time at 12:30 if I can do so. Although we will be picking up the car in Rome we will be driving into Florence from an accommodation about 20 km east of Siena.