The deal is a way for both companies to challenge their primary competitor, Amazon. For Walmart, Amazon has become an existential threat as more Americans have been buying household items online: Amazon's 2015 sales revenue included nearly $80 billion in product sales, and the company accounted for 60 percent of the year's growth in U.S. online sales growth. Walmart's sales, overall, still far outstrip Amazon's at $482 billion, but its e-commerce sales, at $12.5 billion— just 2.5 percent of its total sales—have been slowing in the past two years. In a call with investors regarding Walmart's second-quarter earnings in May, CEO Doug McMillon said that growth in its e-commerce sector wasn't fast enough.