Turbotax usually will guide you as to what to correct. If you don't owe any money; you said tax balance was paid, there is nothing to attach interest or penalties to. I've been doing taxes for a long time; most people believe you HAVE TO file an extension whether you owe money or not. IRS attaches penalty and interest when your taxes are not paid by the due date. If you don't OWE anything there's no need to sweat the extension request.
