I think I would write back and ask him if he has actually paid to join 'ebay plus' as the free postage only applies to members who have paid that yearly subscription fee to ebay, it is an ebay initiative. That ebay then covers the postage. But.. for all other ebay users, that is not the case.
It sounds to me as if he isn't actually a member and may not know exactly how ebay plus works.
If you find he isn't a member, then I would phone ebay to see if you can get the feedback removed but I think they would need to be asked to look at your actual messages because on its own, the feedback is vague, it doesn't mention ebay plus, in fact it praises the shipping. So.. I don't know how successful you would be, but I would politely try to coax an answer out of the buyer in messages to see if they will say why they gave the neg.
Just a couple of points though. I hope you won't mind me saying so, but I didn't think your response to the neg was at all helpful. It is extremely vague too. You don't get the chance to say much so every word needs to count & should cover exactly what the problem was.
As a buyer, if I read that comment, I'd tend to think the fault was with the seller as the buyer said there was fast shipping but unhappy with everything else. That seems like a balanced comment, implies to me that they didn't like the item or other details about the sale and if you then go in with a put down, that tends to confirm the buyer's comment a bit.
I'm not saying that is how it is, just how it might look like.
The other point is that while a neg is hurtful, especially when it is not deserved, as long as your feedback is mostly positive I wouldn't worry. Hardly anyone has perfect feedback if they sell for any length of time. You can never please everyone!
