I have a buyer, but he has been asked to contact me re postage, which I didn't specify, but when I try to do this via "make an offer" it won't accept a combined price greater than the original "buy now" price?
You will need to use 'revise listing' and add a postage option and cost.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.” Søren Kierkegaard, Danish philosopher (1813 - 1855)