Export of services for the year 2017 -18

I am an Accountant.My client told as made export of services for the year 2017-18.But GSTR 3B and GSTR 1 return is NIL for the year 2017-18.But turnover 2.50 crore mentioned in P and L account .Now notice sent tax to be paid from GST .Please advice what proof receive from client.