You can create coupons easily via the coupon management page of the dashboard. Coupon creation is also accessible via the API if you need to create coupons on the fly.
A coupon has a discountType of either of the amount or percentage. If you set it to amount, that amount will be subtracted from the order's subtotal. For example, an order with a subtotal of ¥5,000 will have a final total of ¥0 if a coupon with an amount coupon with a discountAmount of ¥10,000 is applied to it, and an order with a subtotal of ¥20,000 will have a final total of ¥10,000 if the same coupon with a discountAmount of ¥10,000 is applied to it.