[vinay_ys]

There are 5 parties in UPI (or any payment system really): 1. bank customers, 2. bank, 3. payment network, 4. 3rdparty apps (TPAPs) 5. merchants.

UPI is free for #1. It costs #2, #3, #4 to build and operate the systems. So, #5 pays for accepting payments in UPI. But what they pay is far less than what it costs them to accept payments via other means including cash. Also, #2 pays somewhat because it is a service their customer values. Banks also like it because the network fees are far cheaper than other networks like Mastercard and Visa.

Naively someone might think paying and accepting in cash is free. But reality is cash handling can get expensive – leakages (cashier steals), counting and tallying cash, time lost in going to bank branch to deposit cash, dealing with providing exact change etc. are all expensive once you see how convenient and highly productive digital payments is.