Minimum Billing

I am using the master business function B4200310 to add a shipping line to the sales order in F4211.  The problem I am encountoring is that when we turn on Minimum billing for P4210 processing options when this business function processes it flags every order as a minimum billing.  What we have found is that it is only using the amount of the line that I am adding and not taking into account the entire orders line items.  Any idea what I am not passing to the master business function that should be passed ?