I ship about 250 to 300 packages year, mostly books and sheet music,
but also piano rolls.  Most of the packages are sent by Media Mail.
On the roll packages I write, "Player Piano Rolls, Media Mail
173.4.1.e".

Piano rolls are specifically referenced in Section "e" of the USPS
regulations on Media Mail:

  "Sound recordings, including incidental announcements of recordings
  and guides or scripts prepared solely for use with such recordings.
  Video recordings and player piano rolls are classified as sound
  recordings."

Ref. http://pe.usps.com/text/dmm300/173.htm 

Packaging and appearance are important when shipping rolls, books,
or other media.  I use mostly new boxes that are purchased in bulk from
Uline.  I occasionally use recycled boxes but they have to be clean,
in good condition, and with no or very few markings on the outside.

The key is to make the packaging look professional.  An obviously used
box with markings from a toy, electronics or grocery store chain that
is marked "Media Mail" arouses the suspicions of the postal inspector.

Labeling can also be important.  Use a printed label rather than
a hand-written one.  Instead of your personal name for the return
address use "XXXXXXX Music Rolls"

Technically speaking, you are allowed to use a well-used box and a
hand-written label for Media Mail packages, but those are also the
packages which are most likely to be inspected by the Postal Service.

Joyce Brite