Standard Chartered Bank — Credit card wrong calculate late payment interest

Address:

46150

I have been charge rm53 for my late payment in credit card, my outstanding balance is just around rm450 and within 50days i have made all the payment.
But how come they can charge me rm53 as late payment interest as they told me annual interest is just 18%. I have called the customer service to ask about how to calculate the interest since early april 2017, they said will call me again and explain to me about the calculation, but i until now i also do get any answer yet. I have call standard chartered bank more than 5 times during this 3 months period, most of the time they said will call me back within 2 days, but i got nothing! They are trying to ignore customer and not admit they have doing wrong. Very disappointed on them! I wonder is it all the customer also have face the same problem, is standard chartered bank purposely calculate wrong the late payment interest and try to take advantage from the customer?