It's an interesting problem. There's a lot of speculation in this thread, but the general consensus seems to be that it's the USPS's fault: that Esterval ships its packages properly and that the USPS claim that this is either an "improperly addressed" issue or a Customs issue is false.
Assuming that's the case I'm not sure why Esterval would ever take responsibility for packages which go astray. They don't run the U.S. post office (maybe they should), and it's unclear why they would essentially indemnify customers for failures of third parties beyond their control. The only reasons for Esterval to compensate anyone for a USPS screw-up is general kindness, or as a way of continuing to encourage orders from the U.S. In either case there's a limit to the cost they should incur to cover someone else's mistakes. I have no idea how important U.S. orders are to them, but unless the number is a measurable line on their income statement it wouldn't surprise me if they eventually said "screw it, this is more trouble than it's worth." This would hardly be unprecedented; think of eBay sellers or buyers who won't sell to or buy from certain countries. And now perhaps the U.S. gets added to that rarified company.
The obvious answer of course is insurance, but it would have to be general enough to cover any possible loss (i.e. not just lost packages, but packages that aren't delivered for any reason); and frankly I doubt that such insurance is available. And if by some miracle it is, it'd be too expensive relative to the purchase price. And if by some further miracle it's not, the policies would be withdrawn after a number of claims.
As for getting your money back from Chase, my experience has been different. Decades ago cardholders, especially of status T&E cards like Amex or Diners, could count on support in the case of a dispute with a vendor. In my personal experience those days are long, long gone. You can still initiate a dispute, but the presumption of right now favors the seller. And "proving" a case is tougher than ever since many if not most transactions don't involve physical presentation of a card and the creation of a signature for each purchase. That's not to say that charges can't be successfully challenged, just that I think it's tougher to prevail today than it was in years gone by. I'd be interested in hearing how this turns out, although I know it'll take weeks or months to resolve.