Different people are different. For some people mere social interaction is draining even if they are interacting with genuinely nice people.

I'd say rhat the saying is more about saying "thank you" to the cashier and less about giving money

I think you might have misunderstood what being nice means.

It's things like 'it costs nothing to not scream at your waiter because your order is wrong.'

I dunno, maybe it's just how I was raised, but I don't feel this at all. Not being a piece of shit takes zero effort and energy in my experience. It's just the default.

Seems like there's a disconnect in the replies. I think the issue is that some people use kindness to mean the bare minimum and some people use it to mean going above and beyond.

I think the measure is something like if you see someone who tripped and the contents of their bag has spilled onto the floor, is "it cost nothing to be kind" just "allow them some space and don't step on their stuff even if it's in your way" or is it "stop what you're doing and try to help them". Because in case 1 I agree with the sentiment. You're technically doing something (being careful not to step on their stuff), but that really feels like the bare minimum and to a certain extent I'd have questions if someone complained about having to do things like that. In the second case, stopping does take time out of your day and now you're interacting with a stranger who may be busy or distracted and not pleasant back to you. I'd say that's a nice thing to do, but I can see why someone might not want to or not be able to assist with that if they came across it in certain circumstances. If I was in the position of the person whose stuff spilled I would not talk about "kind strangers" who didn't step on my stuff, but would if someone stopped to help. I think the phrase refers to the bare minimum but calls it kindness so people can feel good about not following their basest instincts if they're genuinely rude people.