Don, it appears that you were the victim of an uninformed or "over-enthusiastic" postal worker.
You can check Media Mail requirements here:
Regarding your book and invoice, sans additional advertising materials as Dave mentioned, here are the two sections of their guidelines which indicate your package was in compliance:
Only these items may be mailed at the Media Mail prices:
a. 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.
....and here is the OK for an invoice:
An invoice, whether it also serves as a bill, may be placed either inside a Media Mail piece or in an envelope marked "Invoice Enclosed" and attached to the outside of the piece if the invoice relates solely to the matter with which it is mailed. The invoice may show this information:
a. Names and addresses of the sender and addressee.
b. Names and quantities of the articles enclosed, descriptions of each (e.g., price, tax, style, stock number, size, and quality, and, if defective, nature of defects).
c. Order or file number, date of order, date and manner of shipment, shipping weight, postage paid, and initials or name of packer or checker.
.....and here are the only other things which I could find which would require more postage.....or not. Do you see any issues here:
2.5 Written Additions
Markings that have the character of personal correspondence require, with certain exceptions, additional postage at the First-Class Mail prices. The following written additions and enclosures do not require additional First-Class Mail postage:
a. The sender's and the addressee's names, occupations, and addresses, preceded by "From" or "To," and directions for handling.
b. Marks, numbers, names, or letters describing the contents.
c. Words or phrases such as "Do Not Open Until Christmas" and "Happy Birthday, Mother."
d. Instructions and directions for the use of the item mailed.
e. A manuscript dedication or inscription not having the nature of personal correspondence.
f. Marks to call attention to words or passages in the text.
g. Corrections of typographical errors in printed matter.
h. Manuscripts accompanying related proof sheets and corrections of proof sheets including corrections of typographical and other errors, changes in the text, insertions of new text, marginal instructions to the printer, and corrective rewrites of parts.
i. Handstamped imprints, unless the added material is in itself personal or converts the original matter to a personal communication.
j. Matter mailable separately as Standard Mail printed on the wrapper, envelope, tag, or label.
I'd take the matter up with the manager of your local post office. It looks to me like they made a mistake.