Upvote:4
You don't have to declare anything in Japan.
First you check your luggage in, with the knife inside.
Then you go through security, and finally, just before the passport exit stamp gates, there is small desk with a box where you leave all the receipts you have attached to your passport. Nobody looks at them nor at you, so you don't have to declare anything.
You should declare your purchases in your home country when you arrive, and pay taxes on your purchases done in Japan, but this is a different story.