We are planning on taking the Downeaster back to Boston, then flying home from there (much cheaper tickets than flying out of PWM).
The Amtrak station in Portland is also the Concord Trailways bus depot. There is a bus that runs directly from there to Boston Logan airport with no stops. The Downeaster will bring you back to North Station and then you'll have to take a taxi or subway to the airport. If you've already gotten the Downeaster ride out of your system with the NB trip, you may want to consider the bus SB. But if you're like me, you'll probably want to take the train anyway!
Off topic: Do you know how Portland, Oregon got its name? Portland, Oregeon was settled by 2 men, one from Boston and one from Portland, Maine. They tossed a coin to see whose hometown they would name it after. Portland "won". But it could have just as easily been called "Boston, Oregon." This would have delighted the folks in Portland, Maine, who now have to live in the smaller, less-well known Portland. Back in the days of travel agents, I've actually had them book airline tickets by mistake to the other Portland.