Below is the official regulations for shipping knives and other restricted items as posted in the USPS website:
12.5 Knives and Sharp Instruments
Knives (including sharp-pointed instruments such as stilettos that lack cutting edges) with a blade that opens automatically by hand pressure applied to a button or other device in the handle, or by operations of inertia, gravity, or both, or with a detachable blade propelled by a spring-operated mechanism, are mailable only when sent to:
a. The respective government’s or organization’s designated supply or procurement officers and employees ordering, procuring, or buying such knives for use with the activities of the federal government; the National Guard, the Air National Guard, or the militia of a state, territory, or the District of Columbia; or the municipal government of the District of Columbia or of the government of any state or territory, or of any county, city, or other political subdivision of a state or territory.
b. Manufacturers of such knives, or bona fide dealers of such knives, in connection with a shipment made under an order from any person designated in 12.5.1a.
12.5.2 Addressee Identification
Before delivering a shipment (or parcel) that contains an article or articles described in 12.5.1, a USPS employee may require that the recipient identify himself or herself as in one of the categories in 12.5.1a.
Sharp-pointed or sharp-edged instruments such as knives, tools, ice picks, and razor blades, that are otherwise mailable, must be wrapped to protect their points and edges from cutting through the outer carton in which they are mailed.
As for shipping other kinds of knives, they should only be properly wrapped, as you would any other items like a glass vase or a valuable painting, taking extra effort in making sure that the sharp or pointed parts do not damage or puncture the shipping container.