icici bank amazon in - Amazon Pay ICICI Credit card

Credit card due was on *st May which is Rs.*****.**.and paid the same on **th.May **.There are few transaction done after first statement received.Around Rs.*.** Lakhs are transactioned on **/**/****.In the *nd statement received on **th.May ** with interest amount of Rs.****.**. This *.*% Interest is charged for first amount due of Rs.*****.** and ******.** which is due only on **th.May **.I have contacted customer service and they said since the first due payment delayed for * days and all other transactions which are done in between are liable pay interest of *.*% eventhough the due date for *nd due which is on **th May **.This kind of information is not provided earlier by icici and it is not as per RBI rule which is only ICICI rule. Need help to resolve this interest charges appllied for whole transactions.