Yes, I've never heard of magnets not being allowed in the mail and, while I have never ordered any, I do know someone who has.
OTOH, detectors are getting to the point where you can't hide anything.
In my last visit to China I took a very small Lion battery from a camera. The battery was totally spent and no good any more but I wanted to see if I could find a replacement in China. I put it in a pocket in the suitcase and totally forgot about it.
On the return trip, when I arrived back in Europe and opened my suitcase I found a nice note by the nice police of China explaining they had opened my suitcase to remove some prohibited battery. I had totally forgotten about the battery and it took me a while to figure it out but it seems they have some scanner which detects these batteries and they went directly for the battery I had totally forgotten.
They have scanners that can detect explosives, vegetables, meat, different metals, etc. It is amazing how technology advances these days.