Citibank Credit Card — Fraudulent levy of late fee

In Feb, I underpaid my credit card bill by about 700 Rs inadvertently. Recently when I checked my bills, I saw they had levied Rs 850 as late fine (for 18 days @3.5% monthly ) and 12.5% service tax on this late fee = a total of about 960 Rs on a late payment of <700 -which is more than 150% pm or 1800% per year!!!

On talking to the customer care, I was told that they had levied late fine on the Total amount of Feb bill (44300 approx of which I had paid 43700 by the due date). I do not know till when the case will be resolved- given Citibank's image, but I wonder if people had made trasactions of lakhs of Rs, and underpaid by 10 Rs- how much late fine would they be paying !!!