Does the tracking you have available to you have any means of being able to identify exactly where it was delivered to, such as a photograph of the item being handed over, or being left on the doorstep of the house? If so then send the buyer a reply stating that the tracking shows that the item has been delivered and include a shot of the image that the delivery company provided to show where the item had been delivered to. If the buyer isn't being truthful with you and is just trying to screw you over for a free item then the photographic proof of delivery should prove to be enough to dissuade her from opening an Item Not Received case against you.
If the buyer is not deterred and opens an Item Not Received case against you all you need to do is provide eBay with the proof that the item has been successfully delivered and eBay will close the case in your favour. The buyer will not be able to open another eBay case in relation to the same transaction; however, if she leaves you negative feedback prior to the matter being decided in your favour get onto eBay Customer Services immediately and ask them to remove the buyer's negative feedback in view of the fact that eBay decided the case in your favour. Under those circumstances eBay will more than likely remove the buyer's feedback and the associated defect that the negative feedback would have left.
Finally, if you have not already done so by the time you read this response, add the buyer's User ID to your Blocked Bidders List, making sure that the Don't Allow Blocked Buyers To Contact Me option is activated. That way the buyer in question will be unable to purchase anything else from you in the future and cause you any further problems, nor will she be able to harass you with unwanted messages.
