Citing the postal code section is a good idea. I try to speak their
language and call the materials being shipped as "sound recordings".

Expecting them to dig through the code to find the inclusion of player
piano rolls may be asking too much. That reference is buried fairly
deep; sometimes the less said, the better...

https://pe.usps.com/Archive/PDF/DMMArchive20030810/E713.pdf 
> e. Sound recordings and guides or scripts prepared solely for use
> with such recordings. Video recordings and player piano rolls are
> classified as sound recordings.

Brian Shaw