i agree w/ NWave.. stick to the facts in your reply. respond to what she said, but stick to the facts and avoid getting your emotions involved. if you have to, think like a lawyer!
have you tried emailing her? i once had a similar thing happen, got a neutral w/ no contact. i emailed them and was really nice (very hard to do! ) and said i was sorry they weren't happy, and i wish they would have contacted me first to resolve the issue, as i would have refunded their money if they weren't happy. the buyer was a lil stiff at first, but eventually softened and told me they had had bad exper. in the past, and really didn't think contacting me would help (even tho i state in my auction that i accept returns! ).
and of course the worse part for me was, i had already given them a positive (this changed my feedback policy completely).
anyway, they ended up returning it and added a comment that i was great and they had left the feedback hastily.
you know what they say, you catch more flies w/ honey than vinegar! good luck!!