There are any number of things that can be said that don't come across in writing as snarky.
But, in person, tone of voice, expression (or lack of it) on the face, and a few other clues will let most people (except Sheldon Cooper, Ph.D.) know what is meant...
And even in the Deep South (USA) there are a few times when "bless your heart" means "bless your heart" - if someone has heard of a death, illness, or injury in your family - it is a phrase wishing you well, when you are in trying circumstances.
Same way with "have a nice day" - depending on who says it, to whom, with what facial expression, what tone of voice, and how they are holding their body can make a world of difference. If they are clearly angry enough to throttle you and still saying something polite...well, it might be prudent not to stick around AND be polite as well as speedy on your way to the egress.