Air Asia — Overcharging for providing a seat in the flight

Address:	Jaipur, Rajasthan, 302001
Website:	www.airasia.com

On my recent travel from goa to delhi on nov 28, air asia flight i5 776 charged me inr 400 for providing me a seat in the flight. (pnr # vgwu8j, seat # 7 e & f)
I checked in at air asia counter at 1.30 pm for the flight departure at 3.10 and was told by the lady at the counter that all the seats in air asia flight are chargeable. i was told that if i need a seat from row # 1 to 14, i need to pay inr 200 per seat and for seats from 15 till the end, i need to pay inr 99 per seat.
When i asked her that i have already made a booking months back and why are they charging me for a seat, i was told this is the way air asia works and if i want to board the flight i need to pay this amount. i checked in much ahead of time and there were more than 60-70 customers who checked in after me to collect the boarding passes.
I paid inr 400 for 2 seats over and above the regular booking amount that i paid a few months ago.
While in the flight, i got to know that air asia has charged everyone a different amount. 2 students were sitting beside me who were asked inr 500 per seat for the same row however they paid very less amount.
Air asia is making a fool of customers and are overcharging the customers to provide a seat in the flight. does that mean that when i booked my flight the booking was to stand in the flight and not to sit???
Some lawyer please contact me as i have all the receipts and boarding passes and i want to sue air asia for their fraud they are doing with the customers. attached are the receipts and the card charge receipt reflecting the time (1.31 pm) when i collected the boarding passes.