When I walk in to the coffee shop, I'm free to say "Hello" in one of 10+ languages and no one will think anything of it. Why would I do that? Maybe because I'm in the mood. Ciao
X for doubt. Highly depends on where the shop is based. I would defo get weird looks if I would say Ciao at shops for 'Hello'. But most people will still know what it means. And 'Ciao' for goodbye is actually quite popular in the German speaking countries. But yeah it's not as random as you make it out to be imo.