Amazon Japan charge you referral fees for your sales just like Amazon in other countries. Please see the below for the fees by product category. 

Product Category (Non-media) Rate of Referral Fee
Electronics, PC/PC Accessories, Musical Instruments 8%
(Electronics and Computer) Accessories, Toys & Hobby, Sports & Outdoor, Automotive, Food & Beverage, Health & Beauty, Home Appliance 10%
Apparel, Shoes & Bags, Jewelry, Watches, Home & Kitchen, Office Supplies,
Video Games, DIY & Tools, Baby & Maternity, Pet, Software
15%
Cosmetics (only applies to specific brands) 20%
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