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 -> 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