Question about shipping damage

I recently sold an amp and when the buyer received it he contacted me stating that the "speaker a" binding post were broken but the smp powered up and worked fine. Then I received another message from the buyer stating that one channel went out and the other was distorting.

I had taken the amp to a UPS store to have it packed and I watched the attendant pack it. She did a really good job of packing it with several sheets of bubble wrap and foam and then double boxed it (packing peanuts in the outer box).

The buyer even commented on how well it was packed. I insured it for more than the value so I think I'm covered.

My question is how to proceed from here. The buyer said the box was undamaged so I am mystified as to how the binding posts could get damaged and further more what could cause the amp to distort.

Has any one ever had this happen? If so what course of action did you take with UPS. Should I have the amp sent back and file the claim or contact UPS immediately and then have a pick-up scheduled.

Any direction or words of wisdom are greatly appreciated. Thanks.