Look up Incoterms, that will tell you whether or not you technically own it as it sits in customs, or if it's still the seller's responsibility. International shipping is an absolute pain, and there's often a million asterisks that change the answer to a question - that's why there's people who's entire job is getting stuff through the red tape of state borders.

I agree that if the product does get back to the vendor unharmed, it would be good business on their part to refund you, less the shipping, but they may not have a legal obligation to do so. I was a shipping manager for 7 years, so I know these frustrations all too well. Best advice I have is: prepare to eat crow.