Hey krec, that sucks sorry to hear it. I've had this happen to me using USPS once as well during an ebay transaction and that was using their Priority 2-3day service. After a week I went to the local USPS to see if they could track it down, and they were only able to confirm what I could already see using their mobile app - that it was off the grid lol.
The employees there referred me to the local Post Master right down the road, so I had to call over there, make an apptmt and speak to him. Once there he had access to a better internal tracking system than the local USPS post and was able to see every single checkpoint where the parcel was scanned. There's a code associated with the scan (an employee code I think) at each checkpoint as well that helps tracking. The Post Master was kind enough to print this information out for me and accompany it with a signed letter to confirm that I did indeed ship the parcel in the event that I needed documentation for ebay/Paypal/etc.
Good luck.