i was in the post office the other day for company buisiness and i overheard a conversation with another customer and the clerk on duty, who we like to call "smiling John" as he is not, and the lady was mailing a book. John asked her if there was anything else in the package and she said "well i wrote a note to her", well, smiling John said sorry, no media postage on this, half to pay full price. as you may guess, John does not have much of a sense of humor and is a bit of  a stickler for the rules. so i would not mention the extra piece of paper that we put into the package with the reminder to log receipt of the book.

Under the postal regs that cover Media Mail that slip is considered an invoice or packing slip, and is allowed.

Yup, it's a packing slip Dale...it's all good.  But don't tell 'Smiling John!'