You didn't specify that last posting. Here's the official word
Domestic Mail Manual (DMM)
Media Mail
Qualified Items
DMM E713.1.1a.
Books, including books issued to supplement other books of at least eight printed pages, consisting wholly of reading matter
or scholarly bibliography, or reading matter with incidental blank spaces for notations
and containing no advertising matter other than incidental announcements of books.
Advertising includes paid advertising and the publishers’ own advertising in display,
classified, or editorial style.
ya - I realized that when I reread the post - which is why I included "instructions" in the second 1/2 - and it was clear of any advertising... laugh... It was clearly a book of instructions... anyway she FINALLY believed me... it was a very weird conversation indeed - actually she never did try to get me to go with parcel post - she kept going over the requirements for media mail. It was as though she was not sure what the requirements entailed. It was truly enough for me not to offer media mail again - too much hassle.