plus ebay's 'send invoice' tag is just waiting to be clicked on once the auction is finished, 

I never send an invoice straight away unless a combined discount is involved,  I keep it to send on day three with a polite reminder that if they do not pay up within 24 hours I will be opening a diispute.

As long as the buyer has not paid you can send a revised invoice, as far as I know that is what the fes will be based on, if it isn't I believe a phone call to ebay will sort it out.

I am not selling at the moment and most of my items are free post anyway so I have no experience of the fvf on delivery yet but am basing my answer on what other sellers have posted on the boards.