According to Sect. 508.3.1.3 of the Domestic Mail Manual (DMM):

 

"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."

 

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For further questions (or to report occurrences) regarding flyers being placed into your mailbox without first going through the postal system, please refer to the local Post Office.