Tape the heck out of the boxes. Taping the ends of small flat rate boxes is a must-they will open if crushed. When possible I tape the knife(in baggie) to the box. and always fill the box with packing matterial
I tape the bubble wrapped knife to the inside of the Priority Mail box. Then when closed wrap tape around the length and width of the box. It ain't coming out unless you disassemble the box.
The above is wise advice. Tape the item to the inside of the box, heavily. Make it so the box or padded envelope would have to be completely destroyed to get the item out. Even if the crook gets the item out, the packaging left over would be obviously tampered with (IE, destroyed) and a claim of theft greatly bolstered.
Another thing I have started doing is using a different type of tape under the packing tape. It doesn't have to be a lot, just a piece or two on every seam. I do this to make cutting open the box, taking the item, and taping it back up without notice, virtually impossible. I use some black duct tape I have around the house. Just a few pieces here and there and then tape the crap out of the box with clear packing tape like I normally would.
Unfortunately, in the world we now live in, criminals are everywhere and the are getting more desperate. Fortunately, they are still just as lazy and stupid as always. Most of avoiding being a victim of crime is prevention. With a little knowledge, a few extra minutes, and a bunch of tape, you can make your package look like the least inviting prospect for an easy score.
The techniques I described above I learned here in the GB&U. This is a great place to learn about what to do, and not to do in this hobby. Someone mentioned a sticky or a note about packaging recommendations. While it is a good idea, in practice it may not be worth the effort and space on the page as people already don't read the stickies and rules we have.
BTW, either party can file a USPS claim (and Priority SFRB has at least $50 insurance and maybe $100)
Will USPS pay n insurance claim to someone who did not buy insurance? I have not heard of this. BTW, priority shipping comes with $50 insurance if I remember correctly.