My app has digital content like books, videos, etc., which is accessible for limited time period. User has to pay individually for each content and can purchase again on the end of time period. Thus I am including non-renewing subscription in-app purchase in the app as a payment gateway. We want to offer custom discounts in form of coupon codes to our customers so that they can get discounts on the in-app cost. Is there any other way to do this?