Hi PStacey,
This may sound a little 'off the wall' but I've found that I have had to play 'Poker' with a lot of buyers in the past. I don't think this email would classify as a feedback extortion type, but rather their probably just trying to get you to drop your price or give a partial refund if already completed.
I've had quite a few of these types in the past and my poker method was to always answer in kind first as you did, but after a few exchanges like this with some of them, their emails seemed to get even more abrupt than the last. When this happened, I wrote back again and said, 'That will be fine, since you want to persue this, this way anyways, "Why wait? Lets just exchange these types of feedbacks now and skip the threats if you don't want me to try to settle this with you amicably" (that still leaves the door open for negotiation, your just putting the shoe on the other foot)
I know that these type of response is a little different and maybe unusual? But, although I have recieved a couple of Negs over the last 2 years, never one when I've used any of these techniques.
I also realise that when I do it, with fantastic success, it doesn't improve relations with this buyer in the long run, but who wants them back anyways?
-Jeff