GST PENALTY SECTION 122 - Goods and Services Tax - GST

My client was issued DRC 01 for discrepancies in returns for the year 2017-18 and 2018-19. On submission of clarifications, there was mismatch the input tax credit between the GSTR-3B and GSTR-2A . As the amounts of difference were below 10000 for both the years, in order to settle the issue and buy peace with the department, the same was accepted by the dealer, and the amounts were paid up and DRC-03 relating to the same was also filed. on receiving the order, it is seen that since the said payments are made after 30 days after receiving the SCN the officer has levied penalty 10000 cgst and 10000 sgst for both the years. My query is whether this levy of penalty under section 122 is mandatory even when the dealer has voluntarily agreed to pay the minor amounts to settle the issues ? and whether this penalty can be levied without giving an opportunity of being heard and it is not even initated separately and in the attachment to the order in form DRC-07 under section 73 , it is just made out that the dealer has to pay the penalty under cgst and sgst, without pointing out the reasons for the levy of penalty and when asked they say that in the SCN given it is already stated that if the dues are not paid within 30 days the penalty will be levied without asking. Is this interpretation of the law correct. Is there an appealable case here ? 2017-18 order was pased in jan 24 and 2018-19 in april 24 recently. pl guide what is the way out of this section 122 penalty. thank you.