There are no laws preventing the shipping of a legal fixed blade knife from the post office. His just needs to package it properly and ship it. If the postperson at the post office is refusing it he needs to speak with their supervisor and request to know what section of the postal act he is violating. He is right, they are wrong!

They can however refuse to ship it if he just slapped a few stamps on the blade. It needs to be safe for them to handle!