Transport of goods by road - Goods and Services Tax - GST

Dear Experts, One of my clients is running a Logistics business by transporting goods under the direction of CHA or the customers from the customs port to their factory location. He is running a private limited company. His regular business activity is transporting the goods from one place to another place without entering into any agreement also not issuing any Consignment Note. Further, my client is not charging any tax on the transport charges by assuming he is exempted from GST vide Notification No.12/2017-Central Tax(Rate) dated 28.06.2017 Tariff Heading 9965. Recently, he was audited by the GST department but denied the exemption on the transaction and levied GST @ 18% on the entire turnover along with interest and penalty. I have a doubt, whether he will fall under notification 12/2017 @ Exempted rate (or) 11/2017 @ 18% tax. Please advise me on this, while searching judgment for tax on transport of goods, there are many advance rulings that upheld the rate of tax @ 18% and they fall under Notification No.11/2017 . At the time of personal hearing, the client also stated in their representation no tax is charged and also falls under the exemption category. Please let me know any supporting case law to get rid of this liability. Thanks in Advance.