I found a lot of this info online with a combination of:

tools.usps.com for USPS services, and

Google maps to get me actual locations.

I have personally used the Fort Point office to mail a package just before I boarded an Amtrak train.

The Fort Point location is next to South Station, so, you could take the Silver Line SL-1 bus to.from Logan, although it is rather slow.

The USPS.com site used to be better about telling you if a USPS APC is at an office; like it now says there's an APC at Chelsea, but I'm unsure if East Boston has one. The APCs are very useful for printing that special oversized label you need for a late nite early morning mailing.

MBTA police have been locking up South Station after 10PM to drive the homeless out, so, to reach USPS Fort Point from South Station, you may have to walk outside via Summer Street to USPS.