You have already added a field 優惠碼 in the form for coupon code. You can follow the instructions below to calculate the discount and the net amount. Calculate discount In the Neartail Edit page, click on the Add price field button > select Discount and click Next > select Custom formula and click Next > enter the below formula and click Create. IFS(優惠碼 == "LOYAL15", 0.15 * 訂單金額, 0) Note: Coupon codes are case sensitive. You can change the coupon code and discount percentage as required. You can also add multiple coupon codes or provide fixed discounts. See example below. IFS(優惠碼 == "LOYAL15", 0.15 * 訂單金額, 優惠碼 == "WELCOME10", 10,0) Calculate amount to be paid In the Neartail Edit page, click on the Add price field button > select Net amount and click Next > select Standard calculation using NETOTAL() formula and click Next > click Create. Please try this and confirm.