Anyone have good luck with the USPS finding their lost auto parts in the system?

while USPS sucks for manhandling your package and not taking any corrective effort along the way, it's my belief that your problem stemmed from a poor packing job by the seller and that is who should ultimately be held responsible.

talk it out and see if you can come to an agreeable term between the two of you. set your expectations a little lower and the seller might wind up surprising you.

also, pay for the insurance in the future.
For an individual sending one package once in a while, insuring it might not be a bad idea, but for a business it is just impossible. If as a business you send 3000 packages a month, insurance would easily be over $10 a package, that would be over $30,000 a month and it would either bankrupt you, or you would need to pass the cost to the customer which would make you uncompetitive with the competition. The issue with all this stuff is not that parts get lost or damaged, that is truly inevitable, but the way these companies handle the situation. Just trying to call your local post office, at least in our area, is impossible. You can call the 800 "central" phone number and waste your time, nothing will happen there. UPS after Covid has also made it almost impossible to call them... UPS now has you put a claim and they just give you the $100 "insurance". If what they lost or damaged is worth 10 times that, you are screwed...