RobinB - about your neg - you are absolutely right to give yourself a few days to calm down before responding. Honestly, you will do your seller's reputation more damage by following your initial impulses, than if you consider things from other perspectives.
If I look at your feedback a month from now and see that the buyer gave you a neg, and then saw your proposed response, I would next view the FB you gave to the buyer. If I then saw a retaliatory neutral, I would avoid bidding on your item. Your response would suggest to me that you became very defensive about an error you made. I'd have no way of knowing the history & that you were already trying to work out the problem before the neg was given, but I would certainly sense your anger.
If you simply apologize for the error and emphasize that you wanted to resolve it to the customer's satisfaction in the response area, your neg will look much less damaging to future bidders. You will appear gracious and customer-service oriented.
If you can't bring yourself to leave the buyer a positive, then leave no feedback at all. I totally agree with DMRick's assessment - you may be feeling like the bidder is a witch and is stupid, but the bidder did live up to her end of the deal, which was to pay you within your TOS (I deduce from your description), and then report back to you on her receipt of the item. It was not the buyer's fault that she received the wrong item, nor that you were on vacation and couldn't resolve things more quickly.
I don't mean to seem harsh toward you, and I really appreciate how lousy it feels to get a negative! But certainly consider how this will look to future bidders in a few weeks or 6 months, after your frustration has died down.