If it's the post office screwing up, usually the other person you traded with will respond back to you and try to work something out. Scammers tend to stop communicating.
I've dealt with both.
One trader agreed to the trade. I sent the cards... 30 days go by and nothing back. I messaged them on the trade and waited a full week. No response. Messaged them through their account and waited another week. At this point it was a full month and a half. 3 other traders had already left negative feedback by the time I finally left a negative and it was fairly clear they had scammed us.
In the other case, 30 days went by and before I could message the other person messaged me asking if I'd gotten the cards. I said no so they picked new cards form my want list and sent them with tracking. In this case, even the 2nd package disappeared but because they put tracking on it I went to the post office to look at what happened. I had another incoming trade at this same time that hadn't shown up either. The post office was able to see the package in a video from the last scan but never saw any trace of it after the cart was rolled out of the camera covered area. While they never found any of my cards, the post office did make an arrest at that office for a postal worker caught taking people's mail. And in this case both traders offered to send further replacements, which I said no need since it wasn't their fault, and one of them had already send replacements even.
I recently spoke to another user who seemed to be having the same issue I had in this second case. He mentioned he had 5 or 6 trades all seeming to be missing for several weeks. If it were 1, then scamming is suspected but 5 or 6 all at once... that's a sign something is up at your post office.