Shipping knives in envelopes rather than boxes is a great way to encourage in-transit thievery.
items should be wrapped in a plastic bag and well taped to prevent escape; items that slip around in boxes or envelopes have this tendency to escape. items that don't have sheaths or are otherwise sheathed in flimy cardboard are just asking to have a blade come shooting out the side when dropped (they will be dropped at some point)
a very thin box with padding materials
or item tape TO a sheet of cardboard, then covered by another to make a sandwich, total two sheets of cardboard with item inside; all edges sealed with STRONG tape, then wrap that in a Tyvek type envelope and *address* (for id purposes if main envelope/box/etc opens up) it, wrap 4 pieces of tape again, 2 per side... still not very thick, put that inside the envelope, with a couple pieces of tape inside to anchor it... close envelope, *staple it*. seal it shut using security and/or really strong tape...
the paranoid would take pictures of the major steps as "proof" of technique.
with boxes, i even tape all three dimensions securely with fiber tape...
label - use permanent ink, write large and clear, and COVER the label with clear packing tape long enough to go around package at least once. that way label doesn't smudge, is easy to read, and doesn't "fall off" requiring that package be inspected - which might cause seizure. in any case, a secondary internal label on outside of previously wrapped internal item(s) provide a clue to the shipping people (and doesn't give away the goodies as yet).
package things as if they were the most important things in the world - it takes longer and is worth it, imho. tossing an item into a random envelope and shipping that, well, it's asking for grief. if it takes too long to do, hire someone
Bladite