I'm not sure if this is common knowledge to most of you guys, but I was searching for a parcel number, happened across this court case, and when I downloaded the .pdf for the main case detail, there are pages of a USPS inspector detailing everything they look for on suspect packages, each step of the process, etc.
If you scroll down to the part where it says date filed / description, you can see the "Main Doc" .pdf link.
It's 2018 info but still seems pretty relevant.
Docket for United States v. USPS Priority Mail Parcel Tracking Number (P) 9505 5138 9442 8346 2232 63, 3:18-sw-00330 - CourtListener.com
Docket for United States v. USPS Priority Mail Parcel Tracking Number (P) 9505 5138 9442 8346 2232 63, 3:18-sw-00330 — Brought to you by the RECAP Initiative and Free Law Project, a non-profit dedicated to creating high quality open legal information.
www.courtlistener.com
If you scroll down to the part where it says date filed / description, you can see the "Main Doc" .pdf link.
It's 2018 info but still seems pretty relevant.