Fedex — invoice not charged as per agreed rate card and dimensional weight issues. prelegal notice for invoice overdue

We booked two import shipments AWB NO[protected] and[protected] from China on 22.08.2020 and was stuck in Bangalore custom for more than Two Months. One was delivered to us on Nov 11, 2020 and another was delivered on Nov 2, 2020. But the Invoice was generated before clearance and delivery of goods on 08.09.2020. The Original amount of the invoice they provided was 1, 84, 977.80 INR.

The Original Dimensional Weight AS PER AWB NO is 50*70*65 (29 kgs) & 44*58*51 (20 kgs) but the weight in the generated invoice was 61 kgs (74*71*58) and 34.5 kgs (64*51*53)

Now the problem is The Invoice was not charged as per agreed Rate card which would be only INR 77647.49 (including 18% GST, 17% Fuel Surcharge) for the dimensional weight FedEx provided us (61 kgs and 34.5 kgs) but we were charged 1, 84, 977.90 which was then reduced to INR 82, 049. After this we requested for an invoice split up to know about any extra charges levied and there was no response.

We were constantly trying to get in touch with our account manager Dinesh through calls and emails but is not at all available. Then a Ms Mohana, Representative CFS Team asked us to provide AWB no to look into the matter which we did on Nov 19, but did not get a response from. She then stated that the extra charge shall be adjusted in the next bill. Why? No response.

We then proceeded to contact Fedex Customer Support. Now we have recieved a pre legal notice stating that the Invoice is overdue for 109 days and we need to pay it within 7 days from now otherwise, legal proceedings will be done without further notice.
How is this to be resolved despite us trying to pay the right amount?