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

Click Invoice

Under the "drafts" section, you'll find code to copy and paste. This will be visible to you once you upgrade to paid plans.

This will look something like this :

Click "copy".

Head over to Shopify setting and under Notifications section. Click on "Customer notifications" then click on "Order confirmation.

paste the code and click "preview" to see how it's displayed on Draft order confirmation email. Click "Save" when done.