Square Inc. announced today that it has closed the Series D financing round that included Starbucks Corp., among other investors.

The San Francisco-based mobile card acceptance company Square also said it processes $8 billion in payments on an annualized basis, up from $1 billion a year ago. Its headcount rose to 400 employees, up from 150 a year ago.

Square announced last month that it received a $25 million investment from Starbucks, which also agreed to use Square's software — but not its square-shaped card readers — in stores. Starbucks is also placing Howard Schultz, its chairman, president and CEO, on Square's board.

Square will handle processing for Starbucks, which says the agreement will lower its costs for accepting payments.

A week after Square announced its pact with Starbucks, it announced a new flat-fee pricing option for smaller merchants.

Square's other investors in this round include Citi Ventures and Rizvi Traverse Management. Square says it plans to expand internationally this year.

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