Those saying shipper is responsible to make the claim, I thought so too. But this is directly from usps website. I still say the shipper is responsible for making the receiver whole. You paid them to get a knife back to you.
How to File a Claim
Either the sender or the recipient may file a claim for insured mail that is lost, arrived damaged, or was missing contents. The person filing must have the original mailing receipt. Each claim must be filed within a certain time period and include proof of insurance, value, and damage.