India built the world's biggest digital payments miracle. Now comes the bill

For 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 transactions, potentially ending a decade-long experiment in free digital payments. The government has yet to decide the rate or exactly where it will apply, but proposals under discussion include a merchant discount...