Be very upfront about it, I have found sellers will respond to your problem straight away if you say you will resolve the problem through eBay and leave negative feedback, as no one wants this.... Demand she send the right one express post for Xmas, and to include a postage bag, for you to return her item, and state, that you don't believe you should even have to pay for the bag, as it is clearly their mistake.

I am a seller and buyer and as a buyer have had an item replaced and refunded because of this best of luck