Is there a way to send a canned response to an email when i apply a label to it?

I use labels for task handling, so i have a "Completed" label that I apply to the email once I have finished the task. What I would like is to inform the sender that this task is complete, which I am doing manually at the moment for every single email I tag as "Completed" (and it's a lot of emails per day).

I do not wish to use a 3rd party program as I am very much in love with Gmail's interface