For the first time in three years, the clerk at my local (USA) Post Office did not accept "hobby supplies" as the descriptor on the customs form. He asked for further clarification.
What I wanted to say was "If I wrote down COINS, then they would be swiped in the mail by one of your sticky-fingered greedy opportunistic bottom feeders who clearly aren't getting paid enough on government pay schedules and thus feel motivated to pilfer incoming parcels, just like you did with my 2010 shipment of Swiss francs, so just accept "hobby supplies" and be done with it, okay?"
What I said was "Bus tokens. People will buy anything on eBay."
It happened to me also for the first time perhaps two or three months ago. It seems USPS is determined to stop those evil coin collectors at all costs.
[Unnecessary controversial and political statements deleted by Xavier]
Non illegitimis carborundum est. Excellent advice for all coins.
Make Numismatics Great Again!
It is same story in India - My cousins many posts get returned and stopped. Some they accept send to head quarters where they scan contents and then return to sender after 2 months. This is story from India. My cousin is very frustrated and stopped swapping now for a while until he finds another method. Some he sent to another city and then posted through another relative and so on. Very frustrating.
Singapore - they ask what it is and I always mention "Coins of no value" and they accept, but warn me to register the post. Many times I send normal post to safe countries such as France/USA/UK/Canada/Portugal where coins are not stolen so far. All South American countries I use registered post. But Singapore post did tell me that coins are not allowed to be posted but they said they will close their eyes. So far no problems.
I never had issues when stating "hobby supplies" except the very last time 2 weeks ago when a guy asked me what exactly is in the package. I did say "basic metal coins" but still wrote "hobby supplies" on the form that gets attached to the package. I haven't heard of coins being prohibited from mail especially considering that the mint itself is sending coins coast to coast.
But "tokens" story is another good candidate for an answer next time.
From India all metal sending restricted. So, when I told them, token, key rings, they refuse to take it. They, told me, definitely there is metal in it. Plastic can't be such weight.