I had the same problem.  Here's what I did.Remove the item that is incorrect.  For me it was a pre-ordered item.Update the order.Add the new item.Let me know if this works for you.

Not sure if you're still looking, but we use this app for what you describe:https://apps.shopify.com/orderlogic?search_id=c705412c-9e91-44d0-866d-24268d3f11df&surface_detail=orderlogic&surface_inter_position=1&surface_intra_position=4&surface_type=se...