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Any person - including yourself - could not place anything in a US Postal Service mailbox unless it is related to the US Postal Service.
The USPS holds a statutory monopoly on the exclusive right to put mail in private mailboxes. - http://www.law.cornell.edu/uscode/html/uscode18/usc_sec_18_00001725----000-.html
TITLE 18 > PART I > CHAPTER 83 > § 1725. Postage unpaid on deposited mail matter Whoever knowingly and willfully deposits any mailable matter such as statements of accounts, circulars, sale bills, or other like matter, on which no postage has been paid, in any letter box established, approved, or accepted by the Postal Service for the receipt or delivery of mail matter on any mail route with intent to avoid payment of lawful postage thereon, shall for each such offense be fined under this title.