How can it be a legitimate charge if it's not your car? Possibly your plate number was photoshopped onto a car photo? Must be what happened. If you've got your plates then they can't be on some random car in some other state.
You should send a pic of your car with your plates on it and a copy of your registration to the state or contact them and see what proof they want.
Something is wrong somewhere. Were you even in PA or on the toll roads?
Google this, it is a common scam. Did the demand come from the state or from some random fake site? Maybe you did not contact the state if you contacted them thru a fake email?
Try going to the other state directly.