Two workflows are required for the setup! 

Please check this link with your account name to see if you have 
workflows included in your subscription.

https://account-name.webshipper.io/billing

For Shopify integrations, it is recommended to check the product as a virtual product. 

The first workflow (generates a return label from SKU: 'Returnlabel' when the order is imported):

The second workflow (sends an email to the customer once the return label has been generated):