Hey @TPDEng great question! It would depend on the use case of course, but the simplest implementation would be use Fulfillment Requests within the Fulfillments API. There's more detail here on how to set up a fulfillment management app to manage the functionality, but you could potentially build an integration between the vendor and the shop that could communicate automatically using the Fulfillment Requests API endpoints. From a high level it would look like this for example:
1. An order is created but not yet fulfilled
2. The inventory management app pulls the order and sends a request out to the vendor's "sibling" integration which also can accept fulfillment requests and fulfillment functionality within Shopify (we can also support external fulfillment, but requests are not supported - more info here)
3. Then, if the vendor supports fulfillment within Shopify (to clarify, meaning using our API rather than the process happening off of Shopify completely after the order is created), they can either accept the request or reject it - in both cases they can send a message out, but when rejecting the fulfillment request, it can state which line items it can't fulfill and for what reason.
4. If the fulfillment is accepted then the vendor's fulfillment service app "takes over" the rest of the process and creates the fulfillment along with any notes, etc.
Within the process, it's also possible to cancel fulfillment requests or fulfillments more generally if need be (i.e. maybe an employee accidentally oversold, etc.) - there's more details in the docs share above.
Hope this helps - let me know if I can clarify anything on our end.
Al | Shopify Developer Support
