I have been using
Ebay for many years and have made hundreds of transactions. I have a positive feedback of 573 with 6 negatives. Every single negative I got was in retaliation to my leaving negative feedback for a seller. The ones I did that too really deserved it, mostly for blatant misrepresentations.
You are in a tough spot on the
Ebay feedback front. The bottom line... he can leave you negative feedback for whatever reason he wants and you can do the same for him and
Ebay will not intervene unless the feedback violates
Ebay rules for content. Even then
Ebay will only remove the offending content, the strike will stand.
From what I have read in the posts the guy is either:
A. Lying and trying to take advantage
B. Foolish for paying extra postage instead of pointing out the obvious and stinging from some bad luck on that shipment. Postage due can only come from parcel post that was presented wrong in weight or measurement. Flat rate within rules is flat rate.
If you were to be gracious enough to assume the guy is telling the truth in everything he has reported.... The bottom line is he has had a bad experience with shipping eggs and no matter what you do for him that will not change. If he were a reasonable person and you bent over backwards to help him out a reasonable person would be appreciative and leave you glowing feedback. However, see B above.
Regarding the feedback... he does not sound reasonable and you are liable to see a negative no matter what you do. All you can do is hold back until and unless he does strike against you. I say stand your ground! No honorable person would ever try to extort a person with threats. Take away is weapon, tell him if he feels compelled to blackmail you that you feel compelled to ignore him. Leave the wretch to live with it with no further satisfaction from you.
If he could demonstrate the postage charge I would refund that. That's it. In that event, you would win the morale ground of right behavior.