As a postal employee with first hand knowledge of the FedEx deal and its operations (I had to take a four hour class on the crap), I can tell you that basically FedEx will help ship some (and it is not a big percentage) mail in exchange for money (of course) and placement of FedEx drop boxes in post offices. Postal employees will load FedEx trucks at postal facilities, but FedEx employees will load and unload mail at airports and at the main Memphis hub.
The biggest thing to worry about when mailing gear is to make sure your sources are packaging them safely and securely. Think about what can happen if your package of gear is thrown into a container (literally), and later, a 60 lb box is thrown on top of it. Hopefully, if it is packaged right, nothing happens. However, it doesn't take much for a package to break wide open. I've seen my share of XXX videos, magazines, medicine, and sex toys that have come completely out of the packaging.
I was expecting some gear to arrive the other day, so I checked in the bin with my zip code, and there it was! It was in a priority box about the size of a VCR tape, and it was dented like a big box had landed on it. It was ok, but had the big box landed closer to the sides of the package (where the tabs are inserted to fold the box) it could have been a disaster. Tape should be put over both sides as well as the top where the adhesive part is. That top part can open up easily.
I can tell you that with the volume of mail we get, there is no desire or time for anyone to try to open or "catch" illegal items sent through the mail. As long as you aren't sending a 70 lb box of marijuana or hashish that smells or is leaking marijuana leaves, you'll be ok. Just make sure you and your sources package them securely.