1. Get a lawyer involved. Being cleared by the LEO on scene to get on with your life trumps their idiotic claim.
2. And to those who say you can't sue the government, well, actually you can (may not do any good). But the USPS is not the government- they are a regulated agency, but they are not funded by taxpayer dollars. They are self sufficient from the revenues they generate.

You could get the media involved. Virtually every news station has some sort of protection/advocate team.