So long as you have solid proof that the package was successfully delivered to the address provided when the buyer made payment for the item then eBay will decide the case in your favour. Any money due to you from the sale of the item which is currently being held as a result of the case will be released to you after eBay decide the case in your favour.
If this is just a one-off issue with regards to buyers claiming Item Not Received for goods purchased from you then I would not worry too much about it; however, if this is not an isolated case of things having gone missing recently that you've sent out then it may be worth considering using another courier.