I received a bayonet through the USPS...I was surprised the 'packaging' was still in one peice when I got it. It was about a foot long altogether, and the shipper decided it would be a good idea to tape two smaller boxes together, and throw some newspaper inside.
What I would do when shipping a large fixed blade with no sheath is to glue a few more panels of corrugate on the wall of the box nearest the tip, or if you REALLY want to pull out all the stops, cut a small peice of plywood or louan (sp?) and glue that to the inside. And ship UPS with adult signature required...you might avoid a lot of problems that way (the receivers kid getting it, opening it, bringing it to school)...don't know if it could be traced back to you for the blame, but it coulden't hurt to add the sig required.