Checkout - customer proceeds to order checkout.
Merchant initiates a transaction by sending HTTP POST request to the ENDPOINT URL
Gateway returns a response which contains parameter redirect_url. This is the URL where merchant must redirect Customer's browser to.
Merchant sends redirect to Customer's browser using URL from the redirect_url parameter.
Gateway processes the transaction.
Gateway redirect the Customer's browser to return_url parameter provided by the Merchant in the original request.
Gateway sends a callback with the transaction final status to the Merchant.