The taxi driver will show you the amount in the meter at the end of the ride when you are still sitting in the car. Whether you want to pay him then and subsequently get out and retrieve the luggage, or after you are out of the car is up to you, there is no rule or etiquette, I’ve done it both ways.
Just be aware of the following:
A. Italian taxi drivers accept credit cards but they prefer cash, because they are the biggest tax dodgers in Italy, and cash transactions are not traceable. Some even come with the excuse their POS machine is momentarily malfunctioning, therefore they can’t accept cards (it’s a lie, of course). In your case the charge is small enough that you can pay cash.
B. Italian taxi drivers are not poor. You will not find impoverished migrants doing that job. By law you must be an Italian citizen to hold a taxi medallion (they might be the only native Florentines you will meet since most workers in the tourism industry are immigrants, especially from Eastern Europe). They have a monopoly in the business (Uber is basically banned by a court decision), and have immense lobby power. Taxi Licenses cannot be awarded to companies, only to individuals, therefore the taxi driver is the owner of the license and the vehicle. The average cost of purchasing a license in Florence is between €200,000 and €300,000, but a taxi driver in Florence can easily earn over 500€ a day, even more than in Rome or Milan (well above the average salary in Italy). However the average they report on the tax return is only 15,000€ a year.
C. Because of above points A and B, Italians hate taxi drivers more than Americans hate lawyers and used car dealers. Therefore you will not see an Italian ever leaving a tip to a taxi driver, not even a cent. Sometimes, if I pay cash, I might leave the change (for example if the fare is 24.75€ I might leave 25€ (but not often). They thank me profusely because they would never expect an Italian leaving them a tip. I don’t tell my friends in Florence I left the taxi driver a tip, because my friends will lecture me for hours about the immoral thing I did. Most Italians would like that also Americans kept the long standing Italian tradition of leaving absolute zero tip to taxi drivers. Actually, if you pay cash, you should still ask for the receipt (tell them you need it to get reimbursed by your company). Of course the taxi driver will not give you an official receipt or invoice, he will simply write the amount on a worthless napkin or piece of paper, so that he can continue to dodge the tax man.