Colorodo
...
I'm sorry this happened to you. When your blade gets jacked it's not the end of the world but it can really make you wonder about why some members of the human race are allowed to continue to suck up everyone's oxygen.

Good advice has been given, especially utilizing the Priority Mail mailers. It's a good service overall.
Only thing I will add that may help BF brothers who may be reading this is:
1. Wrap the knife in one of the Tyvek? Priority Mail envelopes like a little burrito and place the whole thing
INSIDE of the Small or Med or Large Priority Mailer and THEN tape <as previously advised by everyone> the "burrito" to the inside of the largest box panel securely. BTW, those Priority envelopes are free at the counter also.
2. Put as many packing peanuts <or other obnoxiously messy packing media> as practical inside the box. When peanuts start spilling out on a dishonest USPS worker like he just hit a jackpot on the nickel slots it just may close the risk/reward synapse in their lizard brain and move them to leave off their larceny.
3. Strategically seal the seams with some kind of whimsical, unique tape that may make a thief think twice about trying to tamper with your parcel because it would be very difficult or impossible to cover up their crime on snap notice. Clear packing tape is too easy to cover up, especially for a dishonest postal worker.
Here's a pic of one of my parcels as I shipped to a BF bro that contained a valuable blade, in "as received" condition on his end:
I've been safely shipping INFI like this for a few years. I've noticed Busse Combat Co. themselves have started using the same tape although it appears they just use it as a novelty because they just use a small "strip" of bacon that's really not strategically placed except for visual effect.
Last point, I've had only 2 blades ripped off from me as a buyer-- zero losses as a seller. The common denominator in both thefts was the blade was shipped to me loose, in an envelope-- no box. They surely made obvious imprints and were easily id'd to actually be felt as to what they were. And both envelopes had razor sharp slits just wide enough to squeeze out the blade. I'm 90% sure it was a USPS employee inside job both times-- and I wouldn't be surprised if it wasn't the same crook both times because both parcels were routed thru the same USPS hub.
Of course if a criminal is motivated and brazen enough, all the tape, peanuts, and bacon tape in the world will not deter them. However, all the advice the other brothers and I have offered add "layers" of security that likely will deter some fence straddlers and keep them on the righteous side more often.
Here's to better luck in the future!