Guess I'll chime in to see if I can offer a little advice....
Having worked for the big brown machine for 13 years, and actually working in the department that deals in lost merchandise I can say it happens a lot. Boxes get jammed on conveyors, torn and items fall out. We catalog the item and send to a main collection warehouse waiting for a claim. Much of it get sold at auction (Kansas City MO guys the main WH is there).
I would say the USPS has some similar process for claiming missing items. Work w/ them, provide photos, part numbers, other identifying stampings the sooner the better before they get to deep in the pile.
I would speak to your local office and ask what the process is to claim lost merchandise, and know th emore information you can give the better. Especially since it is a good bet few at the USPS can tell a clutch cover from a guide bar.
dw