Check the details of eBay's Seller Protection heree: http://pages.ebay.com/help/policies/seller-protection.html

The most pertinent info (should the buyer open a case via eBay):

If a buyer claims an item hasn't been received
You're protected from losing a case if you ship an item within your stated handling time and provide tracking information before you or the buyer asks eBay to review the case.
Tracking information needs to include:
A delivery status of "delivered"
The date of delivery
The recipient's address
Signature confirmation, if an item has a total cost of $750 or more

...so, I suggest uploading the tracking number to eBay ASAP if you haven't already.