You should have not open a claim with the USPS, but instead filed it with ebay or paypal.
I've been on ebay since it started and have a lot of experience with selling, as well as buying.
It's seller's responsibility to insure the item, regardless of if the buyer wants insurance or not. If he did not insure it, then he is an *****. It is also the shipper's responsibility to pursue any claims with the shipping company since he/she is the one who has the shipping receipt, to prove that it was insured.
And again, if it was not insured, it is the seller's problem, not buyer's.
If you open a case with ebay, for item received damaged, you will get your money back, including shipping, AND a return label from the seller.
I always tell sellers - INSURE YOUR SHIPMENT! Regardless of if the buyer "..opted out of it.." or not. Because buyers, most of the time, want to save money, and will say anything to cut their cost. They will "..take full responsibility. blah blah blah..", but if something goes wrong, they will file a claim and ALWAYS WIN.
If I were you, I'd open an ebay/paypal claim, get your money ( which you 1000000% will ), and let the seller chase the USPS/UPS/Whoever for his refund. And instead of hunting for boxes, bases, pieces - Just get your refund and buy another one.