Depending on the state you are living in, some tollbooths have been removed and now charges mount on the car itself through a great electronic toll-collection system called E-ZPass. Sneaky rental companies use this fact to impose surcharges on you over and above the actual toll amounts ($20 for “processing” in one case, $8 in another).
How to avoid: request all the details you can when you rent a car. If you know where you will be driving, talk to the agent about the eventual toll charges.