There are only ticket machines at the airport. The plastic Charlie Card is only obtainable from T staff at other stations. To make the Back Bay Logan Express work for "free" from there, you'd need to buy a T (subway) pass at the machine for multiple rides on the subway and buses. It can be cost effective if you plan to use public transportation a few times. And even more so if you plan to take the Logan Express back to the airport. (Save it, even after its expiry. The driver only glances at it.) Or, sometime during your stay, you can ask a T person at a station for the plastic card.

There's also the free Silver Line from each terminal that gets you once to anywhere the subway goes. All free connections. No credentials needed.

I'm glad to see that ridership has picked up a lot. Word got around.