By Daniel Kline Walmart’s warehouse club had a Super Bowl ad, and it’s following that up with a big offer designed to steal members from rival Costco. Walmart’s (WMT) – Get Walmart Inc. Report Sam’s Club sits in the shadow of its more successful rival, Costco (COST) – Get Costco Wholesale Corporation Report. Both companies follow the same business model. They sell memberships to their warehouses and, in exchange, offer their members limited-frills shopping at very low prices. Warehouse clubs are exactly that — warehouses. Good are packed on pallets, very little staff walks the floor, selectio…