Wow. Its interesting that I found the answer to your question on the USPS website. What a concept!
Can a a flyer/envelope be put it in someone else's mailbox without being mailed? Can I affix a flyer or poster to a USPS® collection box?
Can a a flyer/envelope be put it in someone else's mailbox without being mailed? What if a stamp was placed on it?
Postage must go through the Postal Service™ and be delivered for it to be valid postage and therefore acceptable in the mail receptacle. A flyer cannot be placed in a mailbox after putting a stamp on it unless the item was actually mailed.
"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."
The key to this statement is in the term "postage" which is defined as: payment for a delivery service that is affixed or imprinted to a mailpiece, usually in the form of a postage stamp, permit imprint, or meter impression.