Bummer.
As one who has shipped and received my share of gear, I've experienced a fair number of "rejected" claims. I discovered that UPS states that properly-packaged item can survive a 2 foot drop. This is a good rule of thumb for packing on my end, regardless of shipping method, and instructing shippers on the other end.
It appears that you had a conversation with the seller related to "proper packing" and they did not follow your specific instructions. I think you have a valid reason to request an adjustment from the seller due to the fact that they did not follow your reqquest. If you have an audit trail (via email?) you have a better case.
I once bought a hand-built musical instrument and provided them specifics on how much impact the item would be subject to via shipping. Pack accordingly. It came in damaged. They made an adjustment. I would have preferred that it arrived non-damaged, but atleast the shipper had to pay for their mistakes.
Best luck.