Speaking purely hypothetcailly, one of 3 things could have happened here.
1. it was boxed and shipped that way (by the manufacturer, in essence)
2 It was damaged in shipment, ini spite of there being no visible outer damage, and the shipment being signed-for/acceoted by the recipient
3,. it arrived in perfect condition, and was damaged post-shipment.
Interestingly, no one took the dealer's or the manufacturer's 'side' on this one. I was not party to any part of this transaction or series of events, so I cannot really speak to the facts. Outside of the OP, none of you can either.
As to the notion that 'this is how it is done...' I would ask what the terms of sale were, and whether and what sort of return policy was in place, and who paid for shippng in the first place.When you return something in general, you have to 'bring it back' to the store. They don;t come and get it, and you don;t get reimbursed for your gas or time for the return trip.
Shipping was a service, and was provided. 2 85LB boxes are usually best sent via freight, strapped to a pallet, but that costs a lot more than the usual carrier's ground service.
I am not takins sides here, and hope the OP gets his amp taken care of, but i think we need to know way more detail about this transaction to make more specific claims about who should pay for what.