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.
As the seller, you are responsible for getting the item to the buyer in well packed condition and working and in the physical appearance of the ad. You did the right thing by having UPS pack the item and insuring for full or more value. You need to immediately contact the UPS store you shipped from and get the claims process going. Have the buyer take good photos of everything. All damage, write up a narrative of the problem. Show the packaging--was there any external damage, etc. Make sure the buyer keeps all that. UPS will send the driver to pick up the item with a call tag. UPS has been generally good with shipping damage in my experience. FEDEX not so much. You should be prepared to have this take a while and to stay on it and call daily. A pain but you guys should be fine. Just work together. Good luck!
I don't think they're going to pay for anything, absent a severly damaged outer box. Good luck.
Chayro's probably right. If there's no external sign of damage, UPS is going to deny any culpability. As to the "how" of the distortion, shipping entails the amp being shaken, vibrated, and bumped constantly for the entire journey. Especially if shipped by ground. I've had similar things happen to me three times shipping amps and preamps. No external damage but the product is broken/malfunctioning. I had to "eat" the cost of further repair every time.