So this has bothered me for 3 years! I found an app that calculates it. It's not perfect, ie. you have to manually look up the order and it wont work from my phone yet, but it does this exact thing. Saves me an hour out of my day. They also just upda...

Do you have issues using the Gorgias iphone app. I cannot seem to get it to work? It just keeps me in this login/authentication loop. Very frustrating. 

I just started using an app called Refundably. It seems to work pretty well so far. I'm still on the free trial.