I hope the buyer documented with photos as he received, of the damaged cartons before and as he unpacked them.
You, the person who purchased the insurance, makes the claim. Usually they get picked up from the buyer by UPS, and evaluated at a reginal center.
UPS has a way of denying, they might say that 'double boxing' was needed. You might ask KEF how many pairs of that model were sold/safely shipped in those protective cartons.
You will have to prove their current used value, with original receipts, perhaps some info of used value from hifishark, ....., so insuring for more than their value usually won't get you more than what UPS agrees they are worth.
IF you paid for the shipping with a good credit card, you might be able to use that card's protection, call and ask them. That's why I say, always fund your eBay/PayPal account with your best credit card.
best of luck with this. yes, it's only money, good attitude, but I hope they cover you.

