Elena, it sounds like that person only did one good thing in his entire life: "You were a terrible father, an abusive husband, a lazy employee, but you saved a puppy from drowning - that's your (only) good deed."

++Grumbler

I see. Would it be correct to say "It's a good deed of yours?
As to "that's very good of you" я бы перевела нейтрально, как "Вы очень добры". Но ведь речь о "поступке".??

In this case "that" refers to a specific postupok. Usually we say this after somebody does something, so it should be "that was very good of you."

Clear, thanks !

👍🏼