"That's okay" is a polite way of saying don't do something. It's a polite request. Like when someone wants to bring you something you don't need, you can use it. "It's okay" means "I don't mind you did it or said something". Like after someone apologieses, you say "it's ok".

Things you can use for "That's Okay" when someone tell you this. 1) Is this to your liking ? 2) Is this enough sugar to your coffee ? This you can use for "It's Okay" when someone tell you this. 1) I didn't mean to block you. 2) Sorry that I didn't reply your message for a week, I was really busy.