Exactly what Ocn Vw 1K said. If you return it to the non-agreed upon return location, your rental will most likely be re-priced. Most times, this will incur either a per mileage fee for every mile you had the car (usually $0.40/mi) and/or a drop-off charge (usually in the $50-75 range). If you call Avis before you take the car back, they might work with you. If the location is downtown, it might be a 'co-terminal' where they allow you to drop-off at BOS without a fee - Houston has several locations like that. However, if the off airport location is short on cars, they will charge you a fee because they'll just have to turn around and pay an employee to bring a/your car from the airport.