Not sure where you are, but would it be cheaper to take a car service to Logan? Then you don't have to worry about your car. We've been happy with North Shore Shuttle, if you are on the north shore.
I'll second that. We have used them twice this year without a problem.
The three of us back in 2004 flew out of Boston to Orlando and opted to stay a night at a Boston hotel through www.parksleepfly.com. You get to leave your car there (free parking for the length of your trip), and they provide free shuttle to the airport and then pick you up on the return trip.
We are in Maine and fly out of Logan, and we found it much more economical to rent a car and drive their, and rent another car to drive home. We spent about $80 this way, as opposed to the $144 parking would have cost us.