That’s what I was thinking too. The act of putting it in the mailbox may be a violation of federal law.
On a side note, I believe Amazon and others may have cut a deal that allows them to legally put packages in your mail box though.
Amazon packages are frequently given to the post office for the final leg of delivery which is why they end up in your mailbox. Only the USPS can use the mailbox and they are zealously protective of that privilege.