Maybe there is a quicker way of getting there, but log in, then click on your Summary. Then, click your feedback score right next to your ebay name. Scroll all the way to the bottom, then look for
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Just put in a reply to his neg saying that his negative feedback was unjustified because you fulfilled your obligation as a seller and that USPS is at fault for mis-shipping. People who see neg feedback will read it and understand if they have common sense. If not, then they aren't people worth...