The attendant used 3 layers of thin foam and then three more layers of the large bubble wrap. She placed it in a tight fitting box. Then placed that box into a larger one filled with peanuts.
The problem with this approach is that any blow to the box gets transmitting to its contents. There has to be open space between the inner and outer box so the blow can't be transmitted. So the packing is to blame.
FWIW you were under no obligation to refund the money! This was the shipping company's problem, not yours. It did not help that, with obvious damage, the amp got plugged into the wall and played. There could have been internal damage too.