I think if the shipper can't determine the amount of copper in their products, then neither can customs.

Customs doesn't have to. They can simply decide you haven't followed the rules, and it'll be up to you to prove you haven't or face paying fines/losing a shipment/possible prosecution. And they can decide the playing field: can you be wrong by 10% on that copper estimate? 1%? 0.001%? Good luck.