Shopify lets you use fulfillment services if the warehouse that you’re using is able to process orders by email. Otherwise, you’ll need to develop a custom app with a Shopify API developer. 

To activate a fulfillment service, follow these steps:

Step #1: Go to the “Shipping” section of your Shopify “Settings” menu.

Step #2: Click “Add Fulfillment Service” in the “Custom Order Fulfillment” menu of “Additional Shipping Methods.”

Step #3: Enter the name of the fulfillment provider and their email address in the pop-up form. 

Step #4: Save your changes and add the service. 

When a product or order has been marked as fulfilled, the service will be notified via email so they can fulfill it for you. 

Deleting a fulfillment service is simple. From your “Shipping” page, just click “Edit” next to the service you want to remove, and click “Delete” from the pop-up.

Note: Fulfillment services will not ship items for you until the order has been marked as fulfilled on Shopify.