Insurance, plus registered mail. It may cost a few extra dollars, but if anything goes wrong, it's a chance to at least get your money back.
Despite what people say, USPS isn't all that bad, and it's rare that anything really goes wrong, so you should be ok. You are simply going the extra mile to drop the chances of anything going wrong from one in twenty thousand to one on fifty thousand!
Still, if anything does go wrong and you don't have it sent registered mail, then neither Spyderco nor USPS will do anything to compensate you.
Def