As in all travel, being informed is being smart. Do your research before leaving. Ask the rental agency about transponders, tolls, before leaving the rental agency lot. In Florida, the state's goal is to make all of the expressways "toll by plate." Best to buy a transponder at the first Publix you see. I was the victim of a similar $150.00 fine for making a wrong turn (the sign was covered with snow and unreadable), and entering a cashless expressway near the Denver Airport in a snowstorm. I learned my lesson.