I'm siding with the seller on this really.
Yes, shipping could be combined and really should be, but you paid with it not combined and there is no mention anywhere of him accepting your offer of combining the postage. By paying, you accepted his price which included extra postage per item.
Take a step back and look at it from the sellers point of view.
I beleive eBay charge fees on postage now anyway, so I'm not sure what he's getting out of not including that cost on the listing, other than making it look cheaper than it is.