Indigo Air lines - Wrong GST Invoice Sent

I had booked a return flight for three passengers for the month of April ****. Booking was done in February, after which IndiGo changed our flight numbers as well as timing, several times (even till the last date).
We accepted the changes without issues. However, their GST invoice still has the old flight details, which does not match our boarding passes, due to the schedule being revised from their end. This has resulted in a discrepancy, for accounting and reimbursement purpose.
Upon asking them for a corrected invoice, they are asking me to shell out Rs. ****/- for a travel certificate. This is completely unfair, since the flights were never changed by us, so why should we pay for simply a correct invoice? Furnishing bills with the right data is the bare minimum any company can do for it's customers, and this demanding of additional money is very unfair. I have raised it to their support team, they were extremely inflexible and unhelpful. I also escalated it on social media, yet there is no response from their end.
This has caused me a great deal of pain, waste of time and suffering.