Mobile payments has proven to be a very difficult nut to crack. Google, Isis, and others have failed to bring wallet-free payments to primetime, but one company has seen some success: Starbucks. The coffee chain says that wholly ten percent of its payments in the US are made using smartphones, according to The Wall Street Journal. That's about four million transactions per week. The company has long been the champion when it comes to mobile payments, thanks to its simple apps for iOS and Android, which first launched in 2011. The apps are little more than a digital version of the Starbucks Card, allowing users to reload the gift card and pay using their phone alone thanks to an on-screen barcode. Last year, however, the company partnered with Square to let customers directly pay using a credit or debit card using the mobile payment operator's app. The two have certainly provided some success for Starbucks, but it will likely be difficult to replicate that to other retail locations: the coffee behemoth enjoys a loyal, more tech-savvy crowd than many other businesses.