For those who are interested in citeable information, and performing the actual duties of your position.
DMM 508.3.1.3 Use for Mail
Except under 3.2.11, the receptacles described in 3.1.1 may be used only for matter bearing postage. Other than as permitted by 3.2.10, or 3.2.11, no part of a mail receptacle may be used to deliver any matter not bearing postage, including items or matter placed upon, supported by, attached to, hung from, or inserted into a mail receptacle. Any mailable matter not bearing postage and found as described above is subject to the same postage as would be paid if it were carried by mail.
PO-603 352.4 Mailable Matter Not Bearing Postage
When you find mailable matter in a box without postage paid assessed to, or intended for, the person in whose box it is deposited, take the matter to the Post Office for treatment in accordance with DMM 146.22. If general distribution of the same bill, circular, or other matter was made on the route, return two samples to your postmaster or supervisor with a statement of the number of similar pieces without postage you observed in boxes on the route.
18USC §1725
Whoever knowingly and willfully deposits any mailable matter such as statements of accounts, circulars, sale bills, or other like matter, on which no postage has been paid, in any letter box established, approved, or accepted by the Postal Service for the receipt or delivery of mail matter on any mail route with intent to avoid payment of lawful postage thereon, shall for each such offense be fined under this title.