Air Asia — extra charge for booking seat during boarding pass

I flew along with my family on 10/10/2017 booking number elgv9a booking date 04/05/2017. When i approached for boarding pass i astonished extra rs 240/- per seat, ie total amount 1440/- was charged from me at igi airport t3 reciept batch no i5 1to 2500 no 103947. It was not mentioned anywhere during booking or in my ticket.
Now my question is if i denied to pay the amount what did you do refuse to give boarding pass to me or cancel my journey. If not then why extra amount has been charged. Hope you will reply or refund my money. Niranjan karmakar big member no [protected]. Thanks