An important (for us) category that Dick Kahane left off his list
of media that qualify for USPS Media Mail rates is that of sound
recordings of _all_ types, including, as Craig Smith mentions, piano
rolls.  But "sound recordings" embraces all kinds of music rolls and
book music (oh, my -- heavy!) as well as discs for disc boxes.

Someone recently posted here about an argument he had with his local
postal clerk over mailing discs by Media Mail.  He persisted and won.
The advantage of the Media Mail rate is two-fold: a low per-pound
charge and, more importantly, the rate is not dependent on distance.
You can mail five pounds across the country for the same cost you
would pay to mail it next door.

Matthew Caulfield
Irondequoit, New York