Call the number....I had a friend a year ago do the same thing...accept they threw a dollar in because it was unmanned where they went through and they didn't have change. Got a $52 toll violation/fines, after talking to them got it down to $28...not all that sure what was involved but the rental car agency was part of it.
Basically told them if they had called right after they got home (the number on the website) the charges would have been next to nothing. But they had no idea and felt they were okay cause they threw a bill in
Live and learn...