I have a use case where my website will pull in products/product-options as part of a checkout flow outside of Shopify. The user will ideally pay for the products using my own payment gateway, not using Shopify. Once the order is complete, I would like to tell Shopify via the api that the an order has completed. This message would include the product Ids and the payment-complete status. The idea is that Shopify would maintain inventory numbers. My customers are buying event tickets in my system along with optional merchandise (tracked in Shopfiy). So, people already buying a membership can add merchandise as part of their checkout. As a separate use case, merchandise alone could be purchased directly through Shopify and not through my events system.