Penalty for non-filing of GST Returns

Dear All,

My client got registered in GST in Apr-23.  He sold some goods on Amazon/Flipkart but never filed any return till date.  My questions are :

1. What will be the penalty for non-filing of GST Returns (1/3B/8).

2. Some months have Sale and purchase both and some months have only purchase. Are months having no Sale (Purchase only) will be liable for 10+10 penalty per day or 25+25 per day?

3. Is there any way to save or reduce the penalty?

Kindly confirm.