How becomes a header level charge a line item during autoinvoice?

Hi,

we are working on an issue right now, where, for very few customers, the freight charge somehow doubles during autoinvoicing. These are iStore orders that come in with a predetermined freight charge of, lets say, 50$. Everything looks fine until it gets interfaced. After autoinvoice the invoice suddenly shows a freight charge of 100$ as line item.

To troubleshoot this, I need to understand this process better. Could you please help me how this header level charge is turned into a line item?

Or do you have any other input that might help?

It is not originating from multiple shipments, that somehow got messed up.