For millions of Indians, buying a cup of chai or groceries from roadside vendors has come to mean reaching for their phones rather than wallets – a habit made possible by a system that has so far charged consumers nothing for such everyday transactions.Now, a decade after the government launched the Unified Payments Interface (UPI), India is asking some merchants to pay for the hugely popular system – raising questions over whether it can preserve the low-cost model that drove its mass adoption...