Just a quick update, to whom it may concern:
If you book a Flight ticket (with Indian carriers) 7 days prior to departure, you have the full rights to MODIFY or CANCEL the ticket within 24 hours of booking and get FULL REFUND, no matter what.
This rule is applicable across all airlines that work under the Indian jurisdiction, and operate flights within India. This rule is applicable to not just official flight booking websites, but 3rd party flight booking websites also.
This and many other passenger rights are notified in the Passenger Charter prepared by the Ministry of Civil Aviation, Govt. of India. Download the PDF.
In case any airline denies to provide you refund, you can show them your rights. If they still insists, you can file a formal complaint against it.