HappyEasyGo — Flights refund/credit shell

Due to the pandemic situation currently in india, my indigo flight was cancelled and the airlines planned the amount to be used the in the form of a credit shell.
Now when i wanted reschedule flight using the credit shell amount, i was told by happy easy go to pay for the fare difference amount. Fair enough!

When i received the email of the fare difference amount they asked me to pay was too high, not even providing me the reasons on what basis the fare difference is too high.
Take a look at the below details:

Credit shell amount: 3000 inr
Flight charges :3500 inr
Fare difference amount :2100

And do not give me the reason of blaming the airlines of increase in the price, since i checked with the airlines and the price is not fluctuating it was still 3500!

On what basis is the fare difference amount is charged so much!
For the same they revised the details and fare difference was reduced to 1759 - again with no details provided for the amount charged. And no proper link provided to pay the fare difference amount.

My concern is, when flight booking is so easy, why is it so difficult to reschedule a flight, it takes 3-4 hours for you to provide me a payment link, with no explanation for the fare difference being charged.
Customer care number is told, it is active 24x7! Which is false, i am unable to reach out to them it's been more than 16hours!