Build coupon flows. Every billing platform/provider/service provides coupons, but they never provide what seems like the obvious types of coupons you want, or the flow you would want, unless you are just starting out and have the most basic of coupon needs. If you try to use built-in coupons from your payment processor, after the 2nd or 3rd promotion you run, you are left tacking on custom functionality, writing custom coupon validation, checkout or activation flows, invoicing, or modifying customer account details.