In August, Starbucks invested $25 million in Square and announced plans to let people use the start-up's app to pay in stores.

Customers can now also buy the readers - which in conjunction with a Square app enable merchants to accept card payments - for $9.99 in any US Starbucks and then claim back a $10 rebate when they sign up.

The dongles are already available at tens of thousands of sites belonging to big retailers such as Apple, AT&T, Best Buy, RadioShack, Target and Wal-Mart.