Fedex — Tax Invoice wrongly filed in SB under LUT instead of IGST paid

Address:

307, Shankala Industrial Estate, Gogatewadi, Off Aarey Road, Goregaon East

Fedex company wrongly filed our Tax invoice under LUT instead of IGST paid for the abv said AWB. My gst amount is blocked because of this in customs.
We have been mailing to your team and also to [protected]@fedex.com but we found there is no proper grievance system in your company forget about solving our issue since last 2 months.

1) they never acknowledged the issue. Never accepted the mistake that they have wrongly filed, we have given TAX INVOICE which clearly states IGST paid, have sent mail, still they are not accepting their mistake
2) Also they say SB cannot be amended, which is wrong as UPS has already done an SB amendment to us.
3) toll free nos are useless as they no authority to solve these issues and nor the fedex exceutive mention phones nos of in the mail signature, so where we can speak ?

They only give 1 line answer - SB cannot be amended.