I got once a mistaken transaction on Paypal, the "buyer" contacted me to ask me to refund their money because it was a mistake. I issued a refund and Paypal too

I got once a mistaken transaction on Paypal, the "buyer" contacted me to ask me to refund their money because it was a mistake. I issued a refund and Paypal took 30 cents fees because of the refund. I contacted paypal to ask why I was charged for a mistaken transaction and they just told me this is how it works and there's no way to avoid that.