How do you add a new email to the list of automated email notifications?

Shopify Partner
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One of my store owners would like to that go out when people purchase items.

I found the docs to customise the emails but I would like to add whole email that goes out after the final delivery email is delievered.

I this something I can do from the shopify admin? 

To be clear, I am looking to send the customer the email, not set up a new notification for a fellow staff member.

If this can only be done with an app, do you have any suggestions ?

Thank you.