This is used to Map the Shipping method that you have selected on Etsy with the Shipping method you are using on Shopify. When you created the Shipping template, you provided the Mail Class for the Shipping Carrier you selected. You just need to map that with the Shipping method you are using on Shopify for order fulfillment.
Click on the ‘Etsy to Shopify shipping mapping‘ button on the top right corner to access the shipping method mapping pop-up.
Select the Etsy Shipping method from the dropdown list. Now type in the Shopify Shipping method in the second field. Click on the ‘Map Shipping Method’ button to confirm the mapping.