I ALWAYS use USPS Priority Flat Rate boxes. Find the box at the P.O. Each size has a rate. The small is $5.80 plus insurance costs. You could easily put ten folders in that one, or perhaps six in individual Spyderco or Benchmade boxes. I add some shipping tape on the lid and tabs.
You can stuff the small one or med or large full with as much weight as you can lift, and then insure it for value. I have NEVER lost a USPS Priority flat rate box, and I've mailed all over the U.S., including Hawaii and Alaska several times each. The only way to go, IMO.
Sonny
You could ship them as baggage if you buy a cheap hard plastic gun case for five bucks, that can be pad-locked. Put knives in it. Carry up to ticket counter; tell girl that you are declaring concealed weapons as knives. She will look in the case, then she will ensure that the case is locked with your padlock on it. I've done this with many handguns. Perfectly secure.