Is that the entire sentence? What are you talking about. You can say, 'it's all right' or 'that's all right'. If someone asks you if you want to buy something else, you csn say, 'that's all' but not 'it's all'. I have never heard of 'it's all' by itself.

It is not possible to answer this question without some context. Please provide full sentence examples.

Yes, the proper thing to say is "that's all". :)