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

Wondering if anyone out there has filed a search with the USPS, and has good […] luck

Yup, eventually; see here. But in all those cases the parts were properly wrapped and boxed and there were address labels inside and outside the box. In your case, even if the cardboard held up, USPS is like any other carrier these days: an oily, smelly box full of heavy chunky items is, at best, going to raise suspicion and it's probably going to be isolated and destroyed.