Sending an email only once

I have an orders app. Once the order is complete I mark it with a sent date and the status changes to complete. I want to send an email that the order is done. I've setup a email on the edit orders page which sends when completed date isn't' blank . The problem is if I go back into the order then it will send the email again. I just want it sent one. I've added a "send email" box and added that it needs to be marked as "yes" in order for the email to send. I've also set up a rule that sets it to "no" after the order is submitted and the sent date is not blank. The problem is that it does that BEFORE the email is triggered. Any suggestions that don't involve coding?

What about setting up 2 scheduled daily tasks - one to "Send a custom email for each record" and have the criteria set for Orders that are Complete and "Send email" is "yes"; and, a second one set to execute maybe 30 minutes later to "Update each record" with the same criteria that sets the "Send email" to "No". Would that work?