commerce rules event cart total updated
So you're look to take an action when a Drupal Commerce order's total is calculated. Seems sensible right? There's even a rule for cart updated... but none for total calculated. Okay, we'll do on cart updated. Except... when you add an item to a cart, it updates to add the item, and then updates yet again to update the total! That means you get the event firing once with the wrong total, and again with the right total!
In short... there's just no way to make it work. I failed, and had to research alternatives.
So this isn't very helpful to you seeking an answer, but it is going to tell you now to look elsewhere!