Credit card and applicability of service tax/gst

My client has taken personal loan/ cash loan from Indusind credit card agasint their limit amount before the implementation of Service Tax / GST, at the rate of 14% interest for 3 years tenure and paying EMI regularly. Please advise that Indusind bank credit card can charge the Service Tax/GST @ 18% on the interest portion of the EMI amount as extra from the Customer. and the customer has liability to pay the Service tax/GST on the EMI amount as extra which taken before the implementation of the same. Read more at: https://www.lawyersclubindia.com/Default.asp?