If she filed an Item Nor Received case against you, respond with the tracking number. This will add the tracking information to the case — which, as you say, shows that the package was Delivered. You will win the case. The buyer may receive a courtesy refund from eBay, but you will not be out any money.
You are correct — the package could have been stolen by a porch pirate, or USPS could have delivered it to a neighbor by mistake. The most constructive thing your buyer could have done — knowing as she should that she wouldn't win an INR — is to inquire with her mail carrier or local postmaster about what happened to the package.
BTW, as much as there are sellers who like to complain about how eBay treats its sellers, here is something to consider: I had a case just like yours about 6 months ago, but on Amazon Marketplace . There, seller protection doesn't cover you unless you buy signature confirmation, regardless of value. Like you, I had an angry buyer, not particularly interested in actually finding the package, more interested in extracting a pound of flesh. Amazon stuck a knife in me and drew out $40. By its own rules, eBay won't do you like that in this case, just respond with the tracking number.
