How do I handle special online discounts, coupons, and Promotion codes (Promos) like Groupon, Living Social, CertifiKids, etc, online?

Coupons cannot be automatically applied in online registration. On Tools > Edit Settings > User-defined Fields, you can label a Family User-Defined field as "Enter Coupon information:" which will provide a text box on the Registration form where the customer may enter the information.  Then your staff will manually adjust the posted fee by using the instructions in the main How To section Post a Credit to a Family (Non-Monetary).