NurPhoto via Getty ImagesA vegetable vendor waits for customers at a market in Kolkata, with a QR code displayed for digital paymentsFor most Indians, paying by Unified Payments Interface (UPI) has become almost absurdly routine.Scan a QR code, tap a few buttons and the money moves instantly. There is no card machine, no cash, and - most importantly - for the user, no visible fee.That may be about to change.India has paved the way for banks and payment companies to charge merchants a fee on UPI ...