Returns on Amazon
Most things on Amazon can be returned within 30 days of delivery. In Your Orders (注文履歴), click "Return or replace items" (返品・交換), pick a reason, print the return label, stick it on the box, and send it back.
How much you get back depends on whose fault it is. If something's wrong with the item itself, follow the on-screen instructions and send it back collect (着払い), and you'll be refunded for both the item and the shipping. If you just don't want it anymore, unopened items get a full refund and opened ones only half, and the shipping is on you, sent prepaid (元払い). Opening the outer packaging, cutting off the tags, and opening the plastic bags the accessories come in all count as opened.
Some things can't be returned just because you changed your mind, opened or not: medicine, cosmetics, supplements, food, electric toothbrushes, and magazines are all on that list, and so are Apple products. The same goes for anything the product page says can't be returned, so take a look before you order.
Items that Marketplace sellers ship themselves go back to the seller directly. Their return terms are under "Returns and refunds" (返品および返金について) on the seller's page. If your return request still isn't sorted out after 48 hours, you can file an A-to-z Guarantee claim (マーケットプレイス保証) from the order and have Amazon step in.