I feel you're getting the wrong end of the stick on this one.

You have a 100% rating and I'm guessing that is important to you. eBay can be a pain in the butt at times. Your listing offered 100% Guarantee and a refund if not satisified. Best thing you can do now is suck it up. Have the buyer return everything FIRST then refund him. I would not be inclined to offer a discount if he requests it and he very well may due to the requested phone call vs in writing stored on eBay.

You can eaisly rescale the razor and sell it again. Remember to get a credit from eBay and Paypal for the broken trade.

I would rrecommend removing the 100% Guarantee and refund stuff from your listing. Encourage buyers to ask questions etc but sales are final. Use insurance when mailing razors in case there is a claim.