The supermarket near the main church square shortchanges. There's an old man who clearly hates tourists (he was cursing while dealing with me since I didn't have the exact money).
I suggest when you go to Oia to avoid this store.
I gave 20 and he claimed I gave 10. In my 29 years, I've never had a situation with money before and I didn't even have a 10 euro bill, or I would've given that.
For me, it really ruined my day - not so much the monetary loss but the experience.