We build a private app for our shopify shop to offer a separate check out process. so from the product page customers can add items to our own basket and then "check out" without payment. That creates via API an Order in our shopify system.
Now we face the problem that the app works at our [login to view URL] domain but not at xyz.com. on [login to view URL] it is not possible to add an item to that shopping basket and also it is not possible to create the order