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.
DMM E710.1.5
An invoice, whether it also serves as a bill, may be placed either inside a Package Services 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.