Since w/o a Perform we can't tell if we're on the last line of the order, I was trying to add your item as the 1st line since we can tell if no items have been added yet. Long story short, because of the way V/I processes sequentially line by line instead of consuming all the order data then processing, this is only really possible by inserting the extra itemcode line in the import file itself (but that in itself requires a Perform).
In short, to solve your problem, despite your condition, just like on other SC threads where I've posted straightforward Performs that's been put in use (like how to prevent duplicate A/P invoices), think of this as the same.
Just click on the Perform button on Config tab in Import Job Maint and set it up like you see in this screenshot. Then create the TXT file in the right folder, copy David's code in there but add an EXIT or RETURN as the last line. That's it.