Chief Executive Officer Howard Schultz described the plan as “e-commerce on steroids,” letting customers create standing orders that arrive at their desks daily. The move is part of Starbucks’ strategy to keep U.S. sales growth humming along at a time when consumers are pulling back on discretionary purchases and mobile-phone applications have trained them to seek more convenience. The chain’s mobile app — which lets customers pay and earn rewards points just by scanning their phones — has 12 million “highly active” users, Chief Digital Officer Adam Brotman said Thursday on a conference call to discuss third-quarter earnings.