I saw a setup once where the box & post swayed back and
forth whe mail was put in it. The swaying could be stopped
when the owner retrieved their mail. I once drove down a
street it must have been just after delivery because half
the boxes there were swaying. I thought it was what I had
for lunch until a proud homeowner saw me stopped, and
came out to explain what she and her neighbors had
installed(!) She got her mail out of the box and pressed a
button on the post - the box stopped swaying.