I've shipped things to EC using non-PO-box services by making the address:
3 Ames St., #xxx
Cambridge MA 02142
where "xxx" is the box number where his mail is delivered. Everything seems to arrive just fine using that approach. (Jessie, am I peeving Desk by doing it that way??)
(PS: My bad for mentioning how I sent boxes last year, sorry Jessie! In my defense, I didn't send them until mid-week, knowing they wouldn't arrive until after Rush, since I knew Desk wouldn't want stuff delivered early. If I had it to do over again, I would wait just a couple more days and tell the kid to just tough it out until the boxes arrived.

Thanks for providing the best advice!)