Now on Sale at Starbucks: Square's Credit Card Reader If you're going to be having a cup of coffee with a colleague to talk about starting the next great business, why not grab a way to charge your future customers while you're at it? That's the idea behind a new agreement between Starbucks and Square, the payments start-up. On Thursday, all 7,000 Starbucks cafes in the United States began selling Square's mobile credit card reader for $10. People new to Square can sign up and get a $10 rebate, effectively making the reader free. Square's card reader is a small white plastic square that pops into the headphone jack of an iPhone, iPad or Android device. It works with Square's app to allow anyone, like a dog walker, baby sitter or farmer's market merchant, to accept credit card payments. Square takes 2.75 percent of each transaction. Linda Mills, a Starbucks spokeswoman, said selling the reader was another step to show the company's commitment to Square. In August, the company said every American Starbucks cafe would accept payments from Square's payment app, Square Wallet. Starbucks also invested $25 million in the start-up, and Howard D. Schultz, Starbucks' chief executive, joined Square's board. Ms. Mills added that Starbucks was aware that many people from small businesses come through Starbucks cafes and often do their work there, so offering the reader in its cafes was a good fit.