To add - I agree that payments is great question for systems designs.
Merchants need to receive the money and being able to transact with it. Imagine the merchant receiving the payment is something like Amazon or Ebay (sorry, I don't know what similar large online retailers are in India).
With the above, the problem becomes harder. Imagine receiving 2-3k TPS just on one account during a black Friday or similar day.
Now your system has to perform fine for accounts that do 30 transactions an hour for a retail customer and 3k a second for a merchant.