Since USPS is basically trying to blame byuu for this, I'm afraid the only option here to get any recourse is support-by-public-shaming. It would be a sizable blow to USPS's reputation to be framed as "A bunch of thieves who will deliver your stuff unless it's valuable, then we'll steal it, tell you we lost it, and sell it."
He could try to involve the police, but I imagine it will go nowhere. It's a pity.
EDIT: Corrected 'UPS' to 'USPS'.