Just wanted to put this information out there, because I've been seeing a lot of rumors. One of the things that both buyers and vendors frequently say is that you should never use Priority Express (used to be called Express Mail) because those packages can be opened. This is bologna. If you want your drugs fast, Priority Express is a fine choice. Also, I found a guide to the USPS inspection unit, directed mainly to a LE audience, but is also somewhat ironically helpful to us. If you find anything else, post it in a comment and I'll add it to the OP.
USPS Section 242: Inspection of Packages https://about.usps.com/publications/pub14/pub14_ch2_005.htm
Law Enforcement Guide to the U.S. Postal Inspection Service https://www.hsdl.org/?abstract&did=34409
So U.S. Fish and Wildlife doesn't even need a warrant to open any imported packages. DEA better be making some friends.