One straight forward (and older) way is to charge commission from the retailers. The digital wallets in India started with recharging balance for prepaid mobil

One straight forward (and older) way is to charge commission from the retailers. The digital wallets in India started with recharging balance for prepaid mobiles. When somebody recharges, Paytm and its ilk charges commission in the range of 2-5% from the Telcos.