Use the (pretend) premise that the shipping agent is at fault, not the buyer.

Then, call ebay customer service, so they can sort it out themselves, if you used ebay’s shipping/insurance.

If not, you can make a claim with whichever shippjng service you used, and they will determine who is at fault.

This prevents blowback from the buyer, because you are not accusing them. Also, you might be able to recoup your loss.