I'm fairly certain the original poster is correct - it is ilegal to place in OR on a mailbox - awhile back we had real problems with newspaper delivery near us and I looked into what was and was not legal; for some reason I'm pretty sure ON a mailbox is illegal as well...one moment while I search USPS.gov...ah, here we go:
>>> "...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.gov/text/dmm300/508.htm