Regarding Ed Gaida's message about "Media Mail" in MMD 16.04.04, the
USPS regulation about Media Mail that specifies what qualifies for this
shipping rate can be found at  http://faq.usps.com/?articleId=220395 

As currently written, perforated music rolls are not specifically
listed, although I believe they had been in the past.  However "sound
recordings" _are_ listed, which can arguably be construed to include
our favorite music media.

See also
http://pe.usps.gov/widgets/hyperlinking/highlighted-viewserver.jsp?id=487&title=DMM%20273%20Media%20Mail%20and%20Library%20Mail%20Prices%20and%20Eligibility%20for%20Commer&url=56.42.48.41%3A9080%2Ftext%2Fdmm300%2F273.htm&links=PLAYER%20PIANO%20ROLLS%20#hit0 

It might be a good idea to print this out to carry with you to the post
office since it _does_ mention player piano rolls specifically as being
"sound recordings".

Until further clarified (which I will attempt to do), I suggest placing
a prominent label both inside and outside the shipping container with
wording similar to the following: "Attention: This parcel contains
musical sound recordings (perforated paper music rolls) recognized
under USPS regulations as qualified for shipment via Media Mail.
Package may be opened for inspection."

Note that the package must weigh 70 pounds or less.

John Grant