On more than 6 billion separate occasions in just one month, the ringing of cash registers in India was replaced by audio confirmations on a digital sound box. Add instances of people paying one another rather than merchants, and the world’s most-populous nation drummed up more than 10 billion cashless transactions in August. All were online, instantaneous and cost nothing. At least most of them didn’t. Since April, customers dipping into their mobile phone wallets to settle bills of more than 2,000 rupees ($24) have to bear a maximum 1.1% fee, but only if they are scanning a different platform’s quick-response code. This charge goes from the merchant to his QR code provider — Amazon.com Inc.-owned PhonePe or homegrown Paytm — for hooking up with Alphabet Inc.’s Google Pay. But the Unified Payments Interface, a common protocol for people to send and receive money into accounts at different banks, remains free for regular use.