I tried tipping with card, and they told me that they can't "bump it up" (as in, they will get in trouble if they do). I suppose it depends on the place. I know for a fact that you can't tip with a credit card for parcel couriers. What I do not know for a fact is restaurants. So I suppose it varies. You tip doctors with cash, too. It is illegal to do so, but people do it and doctors expect it, it is just done more discretely.

I was not referring to Poland, but Hungary. What gave you the idea that I was referring to Poland? :P FWIW, I do speak Polish though, and I have many Polish friends.