Upvote:1
It's usually a bad idea to send money by post, because it cannot be insured, so if it's stolen or disappears your money is gone.
However, it's usually not illegal to do so, so if you're willing to take the risk and trust the Polish post, the Swiss post and the Swiss National Bank, you can try your luck.
And in case you were wondering why OP can't just use a regular money changer: Is this banknote still acceptable in Switzerland?