Starbucks' year-and-a-half-old mobile payment program may be the largest of any retailer in North America. Even before its recent $25 million investment in San Francisco mobile payments startup Square, the company had been processing a million mobile-phone transactions per week. The mobile payment options promise a profit boost for Starbucks. By letting its customers make purchases with their smartphones, Starbucks manages to pay fewer fees -- which sometimes top 2% -- on credit-card transactions. Everyone is learning from Starbucks, says Drew Sievers, CEO of mobile payment startup mFoundry, which helped build the Starbucks app and is working on software, due by yearend, for up to nine additional merchants. On Aug 15 more than a dozen retailers, including Target, Best Buy, and Wal-Mart Stores, teamed up to create their own mobile payment app.