If they have bug bounty, then properly report it, and you will be paid by how depth it is and by their bounty program rewards system.
But if they don't have such, i don't think they would care. You might get just a "thank you" without getting paid or getting a reward.

Now if your intention is just to help, you're okay to not getting paid when you report such bug, it's okay, but of if not, i don't mind reporting them as well.