If you want to allow your customers to use our payment portal then you have to do the following:
Submit the order via our API (usual function call that's used for this is POST: /api/orders)
Redirect the user to our payment portal with the Order ID in the URL (Replace [orderId] with your the order id you received from the API)
Demo URL: https://gattinstaging.pei.is/[orderId]
Production URL: https://gattin.pei.is/[orderId]
Contact customer support for access to the production environment