The right way to handle this is if you agree to bid on the item, you accept all terms that go along with the auction. If the seller did not specify combined shipping, then its your responsibility to pay the price you accepted by bidding. On the Chip Board, everyone here usually is very good about charging reasonable shipping fees. However, on EBAY, some sellers consider shipping fees as a part of the total sale. After all, the EBAY model now charges a fee for listing, selling and a percentage of the shipping fees as their commission for handling the sale. If you see one seller has two items you want to bid on in the future, send them an email asking would they combine the total if both auction items are won?
Most will respond in a positive way and others will flat tell you no they cannot combine. In fact, some sellers purposely time their listings so buyers can't take advantage of combining shipping fees. In my many purchases on EBAY and selling, I have been a 100 percent EBAYER by being flexible to represent my products honestly by giving the most accurate description possible.
My selling template always has a shipping discount built into it so there are no misunderstandings. If there is, I usually work to resolve the differences if its the matter of something like shipping discounts, but I take my sales one step further by offering "satisfaction guaranteed or money refunded" in all my listings. If you are a serious EBAYER, your feedback score is primary to your continued success as a seller. Being flexible, offering discounts and communications go a long way in making your EBAY sales both profitable and enjoyable. As for the statement, "should I leave negative feedback," is concerned, the answer is obvious. Why would you expect any other treatment when you read all the conditions put forth in the auction listing.
I know it hurts everyone when some EBAYERs incorporate this form of extracting more dollars out of a buyer than reasonable. In the future, my motto is "If you can't work it out ahead of time, don't buy the item." That way, you have an opportunity to tell the dealer before you bid that his shipping charges are unreasonable, unfair and you won't do business with someone like that regardless if the item you want is one of a kind. Your personal principles are worth 10 times his shipping fees. Good Luck, either way!