Ah yeah... For some reason I read it as free postage killed sales then was listed as a separate cost again and sales came in  (t'was this bit "(after they pay) for the 6 overpaid postage amounts, minus eBay's 10% fees on the postage components for each item." that got me ).

OP - you can either value-add (eg provide a free poster, upgrade the postage service etc), or - if the price of one poster would be around the price of the excess postage - invoice for 6 instead of 7, then cancel the 7th so they get that free? 

Otherwise you'll have to invoice for the total, then refund any overage you want to....unless you send them a PayPal invoice for the correct total. The FVF on the full amount would have already been charged by eBay, so you can account for that in a PP invoice and eBay don't miss out on anything. (If you have a PayPal Me profile, you can also message the buyer the link with the amount after a forward slash).