Most people equate a trip to the post office about the same as they would a trip the registry of motor vehicles.
But a trip to the Kenmore Square Post Office is usually fast and very enjoyable. It's mostly because of the staff who seem like they've been there forever. They know how to manage the different needs (and crazy questions) of people as diverse as an international student from BU to a local-yocal to the snarky meter maid that I see there all the time.
They have PO Boxes that you can rent. Self-service postage machines and scales are also available. If you forget your boxes, you can buy one (even pretty ones to fancy up your package) at the branch.
The one bad thing is that their hours have recently been decreased so they close the doors at 6 PM on weeknights. But they are open on Saturday until 12 noon.