Looking for some feedback from some of you that sell on eBay. I haven't sold a lot but with 726 transactions I have 100% positive rating. Not sure how to handle the following.

2 of my actions ended on May 14th with the same buyer winning. The actions were for Opera House St Patrick's and July 4th sets. I shipped both sets in a small Flat Rate box on May 16th. Entered the tracking number in PayPal and eBay on the 17th. USPS shows the package delivered on the 18th to a PO Box.


June 10th I received a refund request for the St Patrick's set with the following message;
"the PO shows it was delivered but i never saw it. I think somebody there has sticky fingers. I always get an attitude when i go there, I really wanted the chips."

June 11th I replied;
"The chips were sent in the same Flat Rate box as the July 4th set. This box was delivered on May 18th. Was the box opened or damaged? If so you should have filed an insurance claim. Why have you waited over 3 weeks to make me aware of this issue?"

I have not received a reply.

The St Pats set sold for $39.99, the other set for $24.99 and $7.90. PayPal has held $70.47.
My questions are;
What should my next step be?
What will eBay do?
Is my 100% positive rating worth the $40?

Sorry to carrying on like this but would appreciate any help.