Looks like there is now a rearview mirror hangtag transponder:
https://www.frommers.com/tips/car-bu...cashless-roads
Not returning the paper tag incurs a $10 charge. This of course assumes a simple roundtrip. Lots of FL visitors return cars elsewhere, so to be effective, there would need to be collection bins at every airport and neighborhood rental office in the state. And considering that the FL driveout special one-way rates are currently in effect, there should be a provision to mail back the tag.