Do you want to offer coupons to customers who have previously purchased something, or are upgrading? e.g. You could offer a coupon for a customer who has previously bought a different product, and is now buying a higher-value one. Or you could offer an incentive to customers to keep coming back, by giving them a discount for any future purchases. Optionally you can specify a time window for qualifying past purchases (e.g. $10 off your next purchase in the next 4 weeks!).
