Indigo Airlines — Ticket Cancellation took 90% of the amount paid

Address: Bangalore, Karnataka

I had booked a round trip ticket for 2 people.. The total cost was about 8600/-, i had booked it during offer period. Today i had to cancel it for some reasons and the amount they charged is 7000/-.. If penalty is 750 so for 4 tickets its 3000 and include tax and convenience fee, it can still stretch too 4000.. How the hell can they charge 7000 for a ticket which itself was 8600.