Small USPS Flat rate boxes. If it fits, It ships! I take full advantage of this...lol.
When I do a batch of carbon steel knives I wash and degrease them, and get a long length of paper towel and roll all my blades up in it. Lay the blade in, fold over, repeat. I can usually fit like 30 or so small knives very comfortably in a USPS small flat rate box. It's heavy as hell, but it's $5.80 to ship, and it comes with $50 insurance included. Tape the box up super good.
Peters will stack all your knives up together and roll them up with a tight piece of saran wrap. That's how I get mine anyway.