I think tip is uncountable so should be any. Not "a"
My dictionary says tip is countable in all its meanings, it does not make any distinction for the gratuity meaning. What are your sources?
Yeag, I agree, tip is always countable, at least in the US
Are any other translations of the word "tip" in czech?
Yes, but colloquial ones. Díško (or dýško, šort for diškrece, pronounced with hard D), tuzer.