[QUOTE=Johnny Hildo]
I was told by a postal employee that you can’t even include a note that says, “Here is your media” because it would be a violation. I wonder if the same holds true for invoice slips?
[/QUOTE]
Your postal employee may in fact interpret it that way, but their conclusion is hard to reach if you are a layman attempting to decipher postal regulations.
Note this element of the link I posted in my OP:
"4.4 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.
4.5 Incidental First-Class Attachments and Enclosures
Incidental First-Class matter may be enclosed in or attached to any Media Mail piece without payment of First-Class postage. An incidental First-Class attachment or enclosure must be matter that, if mailed separately, would require First-Class postage, is closely associated with but secondary to the host piece, and is prepared so as not to interfere with postal processing. An incidental First-Class attachment or enclosure may be a bill for the product or publication, a statement of account for past products or publications, or a personal message or greeting included with a product, publication, or parcel. Postage at the Package Services rate for the host piece is based on the combined weight of the host piece and the incidental First-Class attachment or enclosure. "