I am going to Orlando for a few days and head down to Key West. It's said that many toll roads in Florida do not have a cash lane. I am considering to buy a Sunpass after landing in Orlando. After checking Hertz website, it mentions that PlatePass in Florida uses the car's plate as a transponder. In NYC area when I don't want to use the platepass I can put the platepass transponder in the iron box. As in Florida the plate is the transponder but I can't cover the plate. If I affix my own transponder to the windshield and go through a Sunpass lane, am I going to be double charged?

Anyone with prior experience please help. Thanks.