It's worth noting that there are a handful of express trips that go directly from BOS to Portland without the stop at South Station. In that case, you end up paying for your ticket when you arrive in Portland.
I typically use the Trailways bus if I need to go to BOS and take the train if I'm going anywhere else in Boston. Of course, starting in Portland means I can purchase a bus ticket in advance and avoid losing my seat at South Station.