How was it sent? Priority? If not, it may still be in transit due to holiday delays. Unfortunately, you are screwed unless the buyer is honest and re-pays you when the item arrives.
It sucks that the ebay/paypal requirement/standard to show proof of delivery is constantly fouled by the incompetence of the USPS. We pay for services such as delivery confirmation and signature confirmation; but many times, by ether laziness or oversight of the carrier, packages go unscanned rendering the added services useless.
The item was sent registered mail (signed for) and I provided paypal the scan of the receipt and detailed document tracking no etc and yet their expert specialist team still decided in favour of buyer with full refund. What happened to "seller protection"?! I did 100% what I am supposed to do