I flew yesterday and checked two bags to get the happy bags coupons. The $20 off coupon is applied at the end of the booking as a form of payment, and it appears to be combinable with other discounts that are accessed by a special link to the flyi site (i.e. the 20% off corporate fares and student fares). The coupon says purchase by 6/30 and fly before 8/1, but I am not sure if the fly-before is enforced by the web site.