Assign Freight Cost at Sales Order - NO Modifier

Dear All,

I'm having a problem and i hope you can help me with it.

I have a Sales Order in which i'd like to add Freight Costs, but i dont want them to be printed on the invoice. There's an option "Assign Freight Costs" at Delivery Tab in Ship Confirmation window, but i need the Costs to be assigned specifically in the Sales Order. Is there any way to do this? I already use Modifiers, but they add the Charge to the Sales Order and also it prints the Charge on the Invoice. The checkbox "Print on Invoice" at the Modifier is actually a non editable field.