That's what the Help section suggested, but I don't have that option in my drop down list (see attached file).

It's complicated, but buyer 1 bought item 1, and when item 2 was purchased, I assumed it was buyer 1 again. But in fact it was buyer 2. (I blame this really bad cold that has laid me low for the last couple of days). Buyer 2 paid before I even sent an invoice. 

So I have already sent an invoice to buyer 1, but didn't include the postage as it had been added to the buyer 2's invoice. I need to add postage to buyer 1's invoice (which happens to be $3 more than the price of the purchased item), so I can't really just let it go.