I've added non-stock item for a surcharge that is dependent on the value of all the other lines on the order.
When I add the line in, I can just set the SellPriceExGST and the cost to the same, and that's fine.
Which value do I update if I want to change the price because the order has changed?
If I just change SellPriceExGST, it applies a discount instead of changing the price?