I don't know about what kind of extra fees may be incurred, but there is definitely an Avis location at Boston's Logan airport. I fly in and out of there all the time, and we rent Avis per our company's policy. You'll usually be driving on the Mass. Turnpike extension into the airport and just follow the signs for rental car return. Once you drop off the car, there's a short shuttle bus ride to the terminal - several different terminal stops so pay attention to which airline gets dropped off where.