Negotiating Shipping costs and Sales Tax under threat of neg

We've seen a couple of these lately.

Buyer wins auction, but requests modification of shipping fee, threatening neg.

This one was a bit twisted, buyer requested modification of sales tax charge.

When seller refused, buyer negged.

Here was ebay's reply to seller's report of "extortion":
Thank you for again writing eBay in regard to your feedback extortion concerns. We're always concerned about policy violations, so we appreciate your report about your trading partner. I have read your emails and taken a close look at the situation you reported. Unfortunately, as stated in previous responses this doesn't qualify as Feedback Extortion. Negotiating shipping charges or taxes are not considered Feedback Extortion. Since a final decision has been made, we will not revisit this case. Although future emails from you will be read about this matter, unless they contain additional information for review, we will be unable to respond.