In Virgina we have "EZ-Pass." EZ Pass allows one to register multiple vehicles per account. When renting, I simply add the rental vehicle info to the list of vehicles in my EZPass account and then use the transponder as normal. When the rental is over I am sure to take my transponder but I leave the rental vehicle in my list of vehicles for about 60 days after the end of the rental. That way, if any charges do come in I can dispute them. Took only one $3 toll with I think it was a $13 service charge from Enterprise to learn this lesson.