Then check the box that says "Send automatic thank you emails for paid invoices." :
The subject of the email will default to the brand name followed by "Thank You For Your Payment".
But you can edit the body of your email. You can add variables by clicking on the gear icon:
And that's it. It's that simple! Now when an invoice is marked "Paid" either manually (or automatically via an integrated merchant account), your client will receive your "Thank You" email.