How to integrate payment gateway in Nepal?

Every gateway follows the same shape even where the details differ. Your server creates an order and a signed request, the user is handed to the provider to authorise the payment, the provider sends them back with a result, and — the step that matters — your server independently asks the provider whether that payment really succeeded before anything is marked as paid. The redirect is a convenience; the server-to-server verification is the truth.

Two failure modes cause almost all of the real-world problems. Trusting the browser's success redirect means anyone who can edit a URL can mark an order paid. And without an idempotency key, a user refreshing the confirmation page or a provider retrying its callback creates duplicate records. Both are cheap to prevent in the design and expensive to untangle once there is money and a customer involved.