You need a checkout system that is some kind of a POS - Point of Sale if you have a brick and mortar business or an E-commerce site if you have an online business. Either one, will have the item/s in the system. Each system should allow for coupons that gives discounts at check out. Many shopping carts can generate the barcode (for the coupon/vauture) and at checkout time, the customer enters their coupon (barcode) and the discount is applied to the whole amount. Once the barcode is used, the system will not allow it to be used again. The same is possible in various POS systems. There are other ways to generate the barcodes, but this would be the best way. Please give more information regarding your current system, and I may be able to answer in a more specific way.