Thats the one. Kind of looks like it is under the shipping label. How could that have happened?
At some point someone thought the shipping label wasn't staying on the package well enough so they retaped it.
The preloader definitely meant to put the envelope on the PC. Those stickers don't come off boxes and get re-stuck on anything very easy. There are a couple styles; some can be retaped easier. Also, the tracking number and address on the tag match the info on the label.
All I can say conclusively is somewhere between when you left the UPS store and when the letter got put on the PC, this label got attached to a letter. Could have fallen off, could have been taken off, could be something else entirely.