IIN that I don't find greeting or thanking people natural?

I sort of get "please" and "thank you" but only to differentiate between asking and demanding. "Can I" works decently though. Anything works that shows the difference between a request and saying, "Give me half of your fries right now" or "I'm taking one of your beers" and such.

"Hi" seems like it should be optional though. I also don't think customers and cashiers should have to ask how one another are or wish one another a good day unless they just happen to actually be wondering how it was, although it's not really their business.

It's not like you're allowed to say anything but "good" anyway which makes the entire thing fake. You're going to look weird if you go buy toothpaste, get asked how you are, and reply, "Fucking horrible. I've been off suicide watch for about a week though. Since then the depression has still prevented me from brushing my teeth but today I forced myself to do something. I will be drinking heavily later and also popping pills, but not enough to OD. Okay. Bye."