I am actually searching for this very same answer and having a hard time finding anything. Have you by chance figured out how this can be done if they’re not on Shopify Plus? Thanks!
Create a private app in your Shopify store’s backend:
Log in to your Shopify store’s backend and go to the Apps section.
Click the “Manage private apps” button.
Click the “Create a new private app” button.
Enter a name for your app and a contact email address.
Scroll down to the “Admin API” section and select the permissions you want to grant to the app. You will need to grant the “Orders, transactions and fulfillments” permission in order to create fulfillments using the API.
Click the “Save” button to create your app.
Use the fulfillment API to create a fulfillment for the purchased item:
In your script, use the POST /admin/api/2020-07/orders/#{order_id}/fulfillments.json endpoint to create a fulfillment for the purchased item. You will need to specify the order ID and the details of the fulfillment, such as the tracking number and the items being fulfilled.
Set up a webhook in your store’s backend:
Go to the Notifications section in your store’s backend.
Click the “Create webhook” button.
Select the “Order creation” event.
Enter the URL of your script in the “Format” field.
Click the “Save webhook” button.
Write a script that listens for the webhook request and uses the fulfillment API:
Use a server-side language such as Node.js or PHP to create a script that listens for HTTP POST requests.
When the script receives a request, parse the request body to extract the order details.
Use the fulfillment API to create a fulfillment for the purchased item, using the order details and the API endpoint mentioned in step 2.
Test your script:
Place a test order in your store to trigger the webhook.
Check the fulfillments section in your store’s backend to make sure that a fulfillment has been created for the purchased item.
Here is an example of how the script might look using Node.js and the Express framework:
We just found a workflow in the Mechanic app that will do this. Mechanic is similar to flow but with a lot more options. We use it to create pick lists from multiple orders. I think it’s $100/month for the app.