Problem with sending coins

Hello everybody, I'm having issues with posting coins.

I am from Serbia and I swapped some coins with Numista member from France. My shipment was returned with explanation that I am not allowed to send coins. I tried another time, but this time I was not even allowed to send the envelope.
Coins were nicely packed and wrapped, they were not visible, they realized what is inside when they opened the envelope.

I want to see if there are members from Serbia or someone who already swapped coins with somebody from Serbia, if there is somebody who can help me with this, how to send coins from Serbia.

I had no problem before, this is the first time I'm not able to send coins out of Serbia.

Thank you in advance

If you wrote "coins" in customs declaration form (assuming one is needed to ship stuff to France) then it´s logical. The other time I don´t really know what happened there.

Yea I would not declare them as coins. Usually I call them tokens or numismatic collectables. I've received package with customs sticker saying metal disks before.

Declare as 'Hobby Materials'

Conceal in padded bag - add some square aluminium foil to disguise metallic shapes

I didn't tell them that coins were in the package, but they have opened it

your coins are found accidentally by customs and returned back (I had this 10 years ago), coins are forbidden to send in all countries. pack again, but send from another post office (everything will be fine).