We arrived at the airport about 1pm on Saturday. I had read posts about the problems returning hire cars with the tank full so was ready for the difficulties.
The last petrol station on the superstrada and the one at the airport were unattended and neither accepted credit cards... cash notes only to be paid before drawing fuel.
With a bit of time on our hands I decided to try and find a place that was either attended or took credit cards. After trying half a dozen I found none that were attended but this one took credit cards:
Gestione Servizi Italia Q8
16 via Conte Fazio
The road runs parallel and just South of the Arno, about a mile from the airport. There's several petrol stations on this road. The Q8 one is on the North side of the road immediately East of the railway bridge. You can see it on Google street view... search Google maps for Servizi & Gestioni Conte Fazio (don't ask me why Gestione/Gestioni has different spellings).
Only slight niggle was that the credit card only got authorised for €69 so having taken this fuel I had to put the card back in for a second transaction for the extra €8 of fuel to fill the tank.