Massport offers complimentary shuttle bus service between airline terminals, Blue Line, and the Rental Car Center. These buses run every 5-6 minutes. It has its routes Shuttle Bus 22 that operates at Terminals A and B to subway station and to the Rental Car Center, and only in midday hours; Shuttle Bus 33 operates at Terminals are C and E to subway station and to Rental Car Center and only in the midday hours; Shuttle Bus 55 operates at all terminals and subway station and to Rental Car Center and operates in the off-peak hours of early morning and late evening; Shuttle Bus 66 operates at all terminals, LOC, Water Transportation Dock, and subway station. There are also additional shuttles with routes: Shuttle Bus 11 that serves all terminals and Shuttle Bus 88 that serves all terminals and also the Economy Parking Garage.
There also give the phone numbers of the airport's shuttle service:
Airport Downtown Hotel Shuttle : 781-475-4717
Here are the descriptions of some of the Car Rental companies: Zipcar is the world's leading car-sharing network. They provide customers with on-demand access to drive cars for one hour or for a day in cities, airports, and campuses around the world. It has a wide range of cars and their prices vary from 14-180$/day. The company has 6 simple rules that the customer must obey. These rules include that customer must keep the car cleaned, throw all trash and take their belongings with them, they should report the damages, or about car condition, maybe it's dirty or there is low fuel; customer should always leave at least ¼ tank, the smoking is not allowed in cars and pets have to be carried in their carriers. And you should return the car on time otherwise you will get $50 late fee. https://www.zipcar.com/pricing Thrifty