We are building a custom app with the Storefront API and we were wondering if it was possible to use the checkout URL from the Storefront API for orders that were not created from the API. For example, if we build a cart inside Shopify itself and click Mark as Pending, are we able to generate a URL from this order thru the Storefront API?
Not sure if this is exactly what you're after (so correct me if I'm wrong), but if you're looking to pre-fill a cart, and generate a checkout link for said cart, you're able to do so via permalinks. You can find some more info on this here and here. Let me know if this helps (or if it doesn't) : )