"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."
Source: USPS Policies
I know lawn & landscape marketing, branding, and web strategy. I'm not anonymous, I just don't self-promote.