(1) it is obviously wrong what the seller did. (It is a waste of time, generally, to ask the seller in advance about combining postage, for I have never seen a seller NOT combine postage. You don't even have to ask after the auction is over as 99.99% of the time the seller will reduce it on his own. it is so obvious (but not here).)
(2) I'd ask nicely for a refund Say it was mailed so cheaply in one envelope, and tell the seller maybe he didn't realize the situation when you asked the first time.
(3) If he says no, I'd tell him to reconsider or else you'd name him on this message board.
(4) I wouldn't leave negative feedback because of retaliation.

Robert