Instead of 28 euros, it is now free. https://www.firenzecard.it/en
Yep, it has been a free extension for quite awhile now; you posted that 8 months ago. 😉
https://community.ricksteves.com/travel-forum/italy/firenzecard-is-back
We used the restart option in March. It worked well but it's not very intuitive so I recommend setting it up before you're standing outside of the next venue you want to enter. After your first 72 hours is over you can 'order' the extension on the app. It's almost like making a new purchase except you don't have to pay, but you will need access to your order number and purchase date (original) and your email because they will send a new activation code.
Unfortunately we happen to be in an area where we weren't getting great cell phone coverage, standing outside of the church we wanted to visit trying to do this on the fly. The person working at the church was very nice and polite, but he didn't know anything about how to activate the extension. It took us about 15 minutes and a little bit of tension, to get through the process but it all worked out.