it will be ok, you need to sell a few more with pos feedbacks, and it will balance back to 100%. Also You can't let this ruin your day! It will go away, but you can't fear or fixate on it. (of course you can, but probably shouldn't).
We love you and Etsy loves you, and besides you can make up, send her a gift, ask her for a return/refund and erase the transaction...etc there are many ways, but the best way to deal with it is to sell more and not to worry about it. I have a cray customer every year, and I know it's part of the whole, it will always happen. Peeps know that 200 loved it and 1 didn't - what does that translates to, so no worries!
One Love!