Make My Trip - Cashback Offer not availed

I made a booking of Rupees ***** (approx) for a international flight on August **, **** using HDFC credit card. Booking id is NN************* While booking, there was a field of promotion code / coupon code which i didnt use as i didnt know that there is HDFC cashback offer going on of *%. I made the payment without applying code as it was not known to me.
But the moment i got to know about the offer from my cousin who is also travelling alongwith me, i called up MakeMyTrip customer service number. This call was made within ** mins of my booking through your website. The executive said since i had not availed it online nothing can be done now. Instead i can cancel the booking and rebook the tickets which is totally illogical. I asked him to connect me to his supervisor. For this he said Supervisor is busy on another call. I asked him to have supervisor call me back immediately after the other call. The supervisor called me after *.* hours once which went missed. I never got call after that one missed call. Hence on **st August i called back the customer care again at **.** a.m. and then again i was told that Supervisor will call back at **.** a.m. Supervisor never called me back. Today after * days Dhruma (supervisor) calls me and tells me it is not possible and gave me an offer as a gesture of **** rupees voucher for my future travel.

I would like to escalate this matter as being a customer i feel i am cheated and duped. When a customer makes a booking, while making the payment , next to the deal code bar there is no option showing me what are the offers available. If i go to any other ecommerce site like Yatra.com / Expedia.com, everywhere when we book a ticket, they show up offers available while making the payment but in case of Make mytrip its not there. A person who is not a regular visitor of websites and doesnt have much time to explore the homepage, will never get any kind of offers if the site doesnt prompt the available offers while making the payment for booking.

In this scenario, i have lost *% of ***** which is **** which i should get as its a big amount. I have already used my HDFC credit card and just because i didn't apply the code of HDFCsale, it doesnt mean i should loose this money. It shouldnt be a matter of code which can be fixed manually with appropriate approvals