You are doing this for the entire order, not per item, so you don't need any custom columns or calculations.
You just need a charge. I have a screenshot of the charge settings I think you could use.Capture.PNG

You are doing this for the entire order, not per item, so you don't need any custom columns or calculations.
You just need a charge. I have a screenshot of the charge settings I think you could use.Capture.PNG
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