My local store wasn't aware of this, but when we found the coupon together online, they gave me the discount for a D30 case. They tried earnestly to also get me to combine this with a "Buy 2 get 1 free" promo currently underway (yes, combine, though the coupon discourages that). Tempting, that.
So whereas normally buying accessories at a carrier store is only less economical than paying MSRP or MSRP plus more at an airport gadget kiosk, these offers can appeal to savvy shoppers. Keep it up, T-Mo!