Seller tried to cover himself in case something happens to the stones. He didn't realize another problem custom fees will hurt the buyer.
The best solution(cheapest) will be. you shouldn't accept the stones and let it return to the seller. Then he ships how you like and you are taking the responsibility for lost etc. early shipping charges you guys split.

I had similar issue 2 years ago.
what happen i did ship goods worth 300.00. buyer never asked me to lower the price etc.
i did send goods with priority +insurance. He couldn't take it because of expenses. fees were really high.
he refused goods and they brought to me back. i ship back just regular mail without insurance and all responsibility was his. He got goods was very happy and paid half of the early shipping.
i thought this may help to resolve your issue.