When I carried mail, we were instructed to pull anything that UPS, FedEx or Amazon placed in mailboxes.
We were to place a notice in the box that the customer had a parcel postage due.
People would get pissed to find they needed to pay postage for something UPS, Fed Ex and a lot of times Amazon messed up.
If someone (usually landscapers) placed fliers in mail boxes, we were to pull those as well. Those we turned over to the Postal Inspectors as revenue theft.
Anything that is properly addressed, regardless of who sent it, was delivered. You can hang all the note on the mailbox you want, there was no stopping it.