What is eSewa API integration?

The flow is a signed handoff. Your backend builds the transaction details, signs them with a secret only the server knows, and sends the user to eSewa to authorise. eSewa returns them to a URL you nominated, and your backend then calls the status endpoint with the transaction reference to confirm the amount and state before recording the order as complete.

The secret must never reach the browser or a mobile binary — anything shipped to a client can be read out of it, and a leaked signing key means anyone can forge a transaction. The other thing worth building on day one is a way to look up a payment by reference from your own admin, because the first support question you will get is about a payment the customer says they made and cannot see.