As per the GST rule, it is mandatory for a business offering sale of goods with a turnover of Rs. 40 lakhs or more to register as a normal taxable entity. And, a Service Provider with an annual turnover of more than Rs.20 lakhs is required to register for the GST. To deal with Legal Notices it requires experience in that field as you are required to give a proper justification for the issue mentioned in the Notice. I would require Email Address and Mobile Number of the buyer in some cases like GST Registration.