Item Charges

Wow - I’ve been getting great help here! Ok, I have a Purchase Order with 5 Lines. Line 1 thru 4 are ITEMS, the 5th line is an item charge called FREIGHT. I have Received/Invoiced Lines 1 & 2, plus Line 5. I did NOT assign the 5th line to any lines. Now I get an error that the 5th line needs to be assigned, or that the invoice number must be zero - which is not possible anymore. Any ideas? Thanks!