Something similar happened to me a while back. I had a couple trades go missing. One of the traders send me replacement cards and included tracking on them. We watch as it made it to Nashua, NH then MIA. I went to my local post office and thanks to the tracking they were able to see exactly where the package was up to the Nashua office where it went into a bin, went off camera and then never showed again. Few weeks later I saw in the news that a Nashua employee was arrested for mail theft so while I never got the cards and the person I traded with ended up out a second set of cards, we were able to catch the person responsible for taking them.
My advice, if you suspect mail theft in your area, offer a few extra cards incentive for a trader to spend the little extra for tracking on the other end, and perhaps you'll be able to catch them in the act.