specifically for tangible media like books,
sound recordings, videotapes, printed music, recorded digital media.
Also known as "book rate," it was created in 1938 by FDR
to encourage the flow of educational materials through the mail.
There isn't much information out about how the service works,
but anecdotally of years with calls to USPS Customers Service
and friends who have worked there,
I know that these packages are used to fill spaces
around larger packages in transit.
The plus side is that they're "piggybacking"
on the routes determined by the larger packages,
so it conceivably uses less resources
and reduces the negative impact on the environment.
The downside is that I've seen footprints on some returned packages.