Add courier tracking info with order completion email

When we change the order status from "Processing" to "Completed", I would like to be able to add some tracking information (tracking number and Courier service selected from a drop down menu). So, I would like a text to be added at the confirmation email like this:

"We would like to inform you that you can track your order using the number ............ . Your order is shipped with ........ ."

Both info fields will be filled with info I will give at the above process of changing the order status.
I am open to hearing any suggestions from you. The point is that I would like to avoid re-contacting the customer with an additional email with the tracking info (except for the order status changing email that he will receive.)