Now Starbucks customers have two mobile ways to pay. Starbucks Coffee Co. and mobile payments company Square are partnering to change the way people pay at the popular coffee shop and help Square expand operations in the burgeoning mobile payments field. Starbucks will invest $25 million in Square as part of the company’s Series D financing round. Square declined to reveal the total amount raised in this financing round. Square will process the credit and debit card transactions at Starbucks’ 7,000 U.S. shops, which will significantly expand Square’s operations while reducing Starbucks payment processing costs, Starbucks says. Starbucks chairman, president and CEO Howard Schultz will…