Head over to "Send to Customer" section of app. Note: you must have pro or higher plans to use this feature.

Click Invoice

Click copy

Next step, Head over to Shopify setting and under Notifications section. Click on "Customer notifications"

Next step,  Click on "Order confirmation"

Paste the code and Click "Preview" to see how it looks. 

Click "Save" if everything looks good!

Note: This won't work for a test order or test stores, it will work on real orders and active stores with plan. Contact us at support@appsfinal.com if you face any further issues