Contents of package switched en route to buyer (by Post Office employee?). What to do?

[scotrk]

RE: http://www.ebay.com/itm/301670412138?ssPageName=STRK:MESOX:IT&_trksid=p3984.m1561.l2649

I shipped 12 loose sleeves (3 dozen total) of new Titleist golf balls to the buyer.  I shipped in a Regional A box to save on shipping costs.  Per my listing description, I was shipping 12 loose sleeves.  It was very clear.

The box was shipped via USPS on July 11th and was out for delivery in Southbury, CT on July 13th.  It came back to the Post Office on July 13th because it was "Undeliverable As Addressed".  It was sent back to the Post Office at Southbury and sat there for a week.  It was finally redelivered to the buyer's home address today, and here's where the story gets very strange.

Instead of the Regional A box that I shipped in, the buyer received a different Flat Rate envelope and inside my items had been switched out.  Inside the envelope that the buyer received was my original invoice and it appears that my original postage label may have even been switched to the Flat Rate envelope as well.  I've been on eBay for about 15 years, and this is the absolute, most bizare thing I've ever seen.

Anyway, instead of receiving my 12 loose sleeves of Titleist golf balls, he actually received 2 brand new dozen of Titleist balls in the boxes!  This isn't even close to what I sent the buyer, and it's a full dozen short.  So now the buyer is short one dozen expensive Titleist golf balls and I'm going to have to replace them at my cost.  What in the world just happened here??  Has anyone ever heard of contents being switched like this?  I'm going to contact the Southbury Post Office tomorrow to see if they've had any other complaints like this recently.  But until then, does anyone have any suggestions for what to do next?  The package was not insured as I've never had an issue with USPS before.  I have 100% Positive Feedback and am as confused as I've ever been

To recap:

1.  sent golf balls in Regional A cardboard box.  Buyer received items in cardboard Flat Rate envelope instead

2.  sent 3 dozen loose sleeves of new balls.  Buyer received 2 dozen new in boxes.

3.  appears my lable may have been reused on the Flat Rate envelope? I can't quit tell.

4.  Package was out for delivery, but was returned to Post Office where it sat for a week before finally being delivered again.

5.  Buyer thinks I shorted him and now I have to make things right

6.  I opened a case w/ USPS National, and the case worked thought this was a very bizarre story as well

7.  I'll contact the Southbury, CT station tomorrow

Thoughts?  Questions? I hope that made some sense.  TIA.