There's nothing new. If the window clerk thinks it's not really media mail they can inspect it.
It's likely your software vendor has started warning their customers as though it's
a change in USPS policy rather than to say, "Oops we forgot to tell you."
Media mail is a subset of the mail class "package services" which used to be called 4th class.
Media mail is only for books, 16mm or narrower width films, printed music,
educational testing material, sound or video recordings, piano rolls, etc.
Regarding an earlier posting about an invoice not being allowed.
That is not correct. One invoice is allowed either inside the package or
in a marked envelope attached to the outside.
The invoice must relate solely to that mailed matter.
ASA, I was just repeating what I was told. Apparently, some of the clerks are also confused. And, when I go to mail them, they ask me if there is an invoice inside. I usually say I don't know; I'm just mailing the packages.