The drop off fee varies depending on the company. Sometimes is $0, lately, like last year, I paid 15€ with Europcar. That is the highest I’ve ever paid, but I don’t know if it increased this year.
.I rent through AutoEurope, but the drop off fee is paid locally, not to AutoEurope when you purchase the contract (generally a credit card charge debited to your card when you return).
Montepulciano is substantially further south than Florence (at least 1.5-2 hours on a good day without traffic), therefore in your shoes there is no way I would backtrack to Florence.
Your better options are:
Return the car in Chiusi, at the Chiusi Chianciano station, and take the train from there. Chiusi is very close to Montepulciano. This option would minimize driving, however be aware that the only option in Chiusi is Avis-Budget, which has an office near the Chiusi station. The office is a small franchise, therefore they are open Monday through Friday and Saturday morning only. They also observe a midday break at linch time.
Drive all the way to Sorrento and return the car in Sorrento. Most major companies have an office there. They are not large offices, so they observe the opening times similar to Chiusi.
I use AutoEurope (a broker/consolidator) when I rent in Italy. Give it a try. Enter as pick up location FLORENCE ALL LOCATIONS, enter as drop off location CHIUSI or SORRENTO. Obviously the Chiusi option will display only Avis-Budget deals.
Another drop off option, if you return it on Sundays, is the Naples airport, which is always open. However you then need to incur the extra time and expense from there to Sorrento.
