There are US limits on exporting certain firearm parts, and a scope meant to be used on a rifle IS one of the restricted items.
You are going to tangle not just with Japanese law, but US law. Dept of Commerce sort of thing. Do NOT think you can drop it in the mail. It WILL get picked up at customs, and you will not see it again. As in sayonara.
In the words of Willy Wonka-”I wouldn’t do that. I really wouldn’t. No… Stop… Don’t.”
What we have here is... failure- to communicate.