The Late Fees shall be be Rs. 50 per day ( Rs. 25 under CGST + Rs. 25 under SGST ), incase of "Liability Return" and in case of "NIL RETURN" it will be Rs. 20 per day ( Rs. 10 under CGST + Rs. 10 under SGST ). However the Maximum Late Fees that can be charged per return per month is Rs. 5,000/-
The GST Amnesty Scheme ended on 30th September, 2020. So you cannot avail the benefit of Rs. 500 per Return per month.
The due date to file FORM GSTR-3B is 20th of the following month. Incase of Small Taxpayers ( having aggregate turnover upto Rs. 5 crores ), the due date is 22nd and 24th of the following month for Category I and Category II States respectively.