The best insurance is in the precautions you (or your seller) take in packaging and shipping the item being sold.
The Allnic T1500 amp has a shipping weight of 48 pounds and appears to be well constructed but fragile with respect to damage that could occur upon an impact.
Original mfgs. packaging is usually ok for lighter items but, amplifiers, monitor speakers, and expensive electronics, IMO, should always be double boxed inside of a sturdy, heavy duty cardboard, or wood, outer box with at least an inch of foam or other impact damping material (not peanuts) between the inner (mfgs.) box and the outer box. The mfgs. box should have sufficient inner packing material to keep things in their place if (when) they are jostled about but, the outer box is the one that should be designed to take a hit and, along with the one inch of foam between the boxes, to safeguard the mfgs. box in the event it is dropped.
IME, any type of floorstanding speaker that you cannot afford to have damaged should be strapped to a pallet and shipped by a trucking company.
It has long been my belief that the seller owes the buyer the purchased item in its as-described condition and that the item is the seller's responsibility until it arrives safely to the buyer. As others have pointed out, the shipping industry supports this in that the seller/shipper pays the shipping company for shipping and insurance, and claims are made by, and paid to, the seller/shipper, not the buyer/receiver. The only control the buyer/receiver has is to review the packaging upon delivery and, if badly damaged, either not accept delivery or arrange to open and inspect the item in the presence of an agent of the shipping company to look for obvious damage before accepting it. Another consideration is that (I have been told) UPS/FedEx have certain insurance limits (I think maybe $5K) above which they internally require special inspections and presumably apply extra scrutiny to the drivers to take precautions. For items valued near or above the threshold amount, paying the extra insurance may provide you with extra care by the shipping company and the driver/delivery person.
I understand that, in this case, the horse is out of the barn already, and I am sorry for your troubles, but the above practices have worked out well for me and I thought might be beneficial to the discussion.
One final thought is that the seller's attitude and response to you are unacceptable here so if, after an honest attempt to give the seller a chance to own up, I agree with others that you should file a complaint with A'gon and take whatever other avenues you have to rectify the situation.