on the same invoice both CGST and SGST is to be charged. HSN code is optional i.e not required to mention HSN Code in their invoices for Taxpayers whose turnover is below Rs 1.5 crores , turnover above Rs. 1.5 crores but below Rs. 5 crores shall use 2 digit code and the taxpayers whose turnover is Rs. 5 crores and above shall use 4 digit code.