Ok, so yes, you will need a little more configuration for this to work as you want it to, because when you add 1 item at a time, it will not change the price of any of the previous items.
You will to need to use Order States and discounting Order Tags to accomplish this, rather than trying to use a simple Price Change. The setup is quite a bit more complex, but it can be done… I know this from some things I learned when putting together my Happy Hour and VIP Discounts.
I can try to put something together for you, but it won’t be today. So for now, you can try to study the methodology that is discussed in the above Topic. When you understand how it works, you will understand why you need to use States and Tags to get your desired behavior. When you use States, you can check for Order State Changed Event and then apply Order Tag Discounts based on your chosen criteria, and it will “loop” through previous order lines accordingly and apply or remove the Tags.