Where do they get their store ID from? And presumably the customer needs your app. How does the merchant know they have been paid?

Stores apply for a business account with Safaricom -- the owner of the Mpesa mobile money network in Kenya. Safaricom assigns them a 6-7 digit till number, to which customers can send funds from their own mobile money accounts. Yes customers would need our app, or eventually the PWA web version. Our app gets a paid receipt string back from the mpesa service which we display in the app once funds are sent, and on the merchants end they see the same receipt string in their POS terminal. Customers will typically show that string to the cashier who verifies it matches the string (& amount) they were sent from Mpesa, and if they match, transaction is completed.