I thought the poster was implying that the USPS was enforcing ownership even though they didn’t create or provide the content, so making your own mailbox wouldn’t help at all.
Yes. If I make an exact (3D model) replica of a mailbox, or analogously, a re-recording of a song that sounds exactly the same, the original designer of the mailbox or song can exert authorship rights through copyright action.
The fact that you did the effort of making the mailbox in a drawing program or re-recording the song doesn't mean its suddenly free of copyright.
