Go to the Sales → Orders → Edit Order
Save some additional order details, such as payment results, and add notes to the order for customers to view. Some payment gateways also add notes for debugging.
In the Status & Comments page the store owner can change the status of an order, type a new comment to the customer to read when logged into their store account, and by checking the "Notify Customer" and "Append Comments" checkboxes the customer is notified via email and a new comment is added for tracking.
Notify Customer - If the customer is notified, they will receive an email in the format of the order confirmation email.
The notes feature can be a very powerful tool for communicating with customers. Need to add a tracking number for shipping? Some stock is delayed? Add a customer note and they will be automatically notified.