South Indian Bank [Sib] — Credit card - interest related

Address: Gowreesham,TC55/2116(3),Kalpakagardens,Pappanamcode,Thiruvananthapuram.695018

I have been using one Visa Signature credit card from October 2022 onwards. Credit card bill is generated from 16th to 15th of next month and the payment date is to be made within 3rd day of next month. There is only one bill generated for both credit cards.
For the Bill payment of December 2022, there came a delay of 8 Days. For the next month's bill an amount of Rs.2899.98 is taken as interest and Rs.310 as fee and charges. I have paid the entire amount as stated in the bill regularly and promptly without any delay as per the bill statement and after payment, the bank both in one card App and in the message shows that the bill was fully paid. On going through the previous bill statement I came to know that the bank is charging more than Rs 2500 every month as interest. I filed a complaint the customer care of one card by email. They contacted me and said that the interest amount will always to be charged to every bill as I had made a delay of 8 days in paying the bill. They also told me that as per their policy even though interest has been charged and paid for the delayed payment in every outstanding amount of fresh purchase of both cards in every month the interest will be levied.
The interest calculated and collected from me is high and excessive. For the fresh utilisation of both credit cards in this month that is October 2023 also I am constrained to pay more than Rs2500 as interest for the delayed payment of 8 days of December 2022 which is unreasonable arbitrary illegal and violation of banking policy.
So kindly look into the matter and make necessary steps to reimburse the amount which was illegally collected as interest from me with interest and compensation