Then I do not see anything too complicated. When you buy knives simply tell store person that you are planning to ship this via mail. Likely they will put the content into the appropriate box. Most stores in Japan will do that for you if you ask. Then simply ship it out from a post office. I do not see any complexity involved as far as shipping is concerned. I have sent items between the U.S. and Japan many times.

Since knives seem to be not a prohibited or restricted items as far as shipping is concerned, it should be no different than shipping any other package.