It is illegal to place in or on USPS mail boxes anything then standard US mail.
Any advertising on a mailbox or its support is prohibited.
Except under 3.2.11, Newspaper Receptacle, the receptacles described in 3.1.1 may be used only for matter bearing postage. Other than as permitted by 3.2.10, Delivery of Unstamped Newspapers, 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.
http://pe.usps.com/text/dmm300/508.htm#wp1051804
Forensics/CSI
Sworn Expert Witness in Criminal Law. Sworn Expert Courtroom Testimony.