If you hear from them again, you need legal assistance. Follow these 3 simple rules:
1. shut up
2. Shut Up.
3. SHUT UP!!
IIRC, postal inspectors are full-blown federal LEO's, with arrest authority, etc. If they can twist whatever you say to appear to be less than truthful, you can be charged with obstruction. If they come again, politely tell them you have nothing to say without the presence of a lawyer. Then get one on the double.
As federal officers, they've got to do mucho paperwork. If this investigation was for real, and not just a bullying session as a favor to somebody further up the chain, there will be a report on it. Try to get a copy of their report under FOIA. Because it may be characterized as a criminal investigation, though, you might be denied access. Nonetheless, it doesn't hurt to try. If you are successful, don't be too surprised to see "uncooperative" appear several times; in my experience that's LEO speak for "asserted constitutional rights and refused to answer or lawyered-up."
Hopefully this little episode is over; kudos to you for putting everybody in their place!