Here’s how to send customer emails when you’ve processed your customer’s order.
When you look at an individual order, you will see a box on the top right that says “Order Actions”.
Use the dropdown box to choose Completed Order and click the little arrow next to it to send the customer that email. Once you are done, click the Save Order button below so that the orders page will show the status of that order as completed. That will make it easier to know which orders have been processed in the backend.