Air India Airlines - Extra baggage charges

For my flight from Bangalore to Singapore on **-**-**** via flight number AI ****, I was charged extra baggage fee for * kgs @USD ** per kg. When I did my booking I tried to book extra baggage but the option was not available online. I tried reaching customer service on Air India hotline and customer service officer advice me that the extra baggage charges for International flight from Bangalore to Singapore is Rs*** only. Then I asked him to allocate extra ** kg for my ticket, but the officer advice me that he cannot do the same from his end and this extra baggage of **kg can be taken from the counter on the same cost of INR ***/kg at the airport.
On the date of travel, I was shocked to find that airline is charging USD ** per kg instead of INR***/kg which was advised to me by the customer service officer. I felt cheated as I was told that the charges will be Rs***/kg for International flight from Bangalore to Singapore. Had I been told that the charges would be USD**/kg, I would not be caring the extra baggage. I therefore request you to refund the extra amount at the earliest.