Upvote:5
Airlines reserve the right to change or cancel flights and spell out their responsibilities in the terms & conditions of your booking. But 99% of us don't bother reviewing this, we simply buy the ticket and check whatever boxes they tell us to. You should read their terms & conditions and Contract of Carriage first, before deciding to spend money on a lawyer.
Why did the airline cancel your flight? Weather, mechanical issue, route being dropped? Depending on the reason, your avenues of recourse may vary. But in the long run, they cancelled, they offered a refund, end of responsibility.
What is "100/- convenience charges" (I am guessing the first is 100%)? So what convenience charges are they not refunding? If they cancel they should refund airfare, any advance paid baggage, seat or food charges. Charges such as a call center agent fee (which some carriers now charge) would not be part of this, as you got the service you paid for. A caring airline would likely refund this, but they wouldn't be required to.