Ekart Logistics — Complaint

I had couriered a package from Mumbai to Karnataka. I am the seller. The package was COD. The buyer received the package but your courier guy marked it as undelivered, took the money and kept with him. Also he asked the customer to remove the sticker which has the details and the AWB number so i can’t file any complain. The customer had no idea about it and give the sticker unknowning.
Now the courier guy is ignoring me and i have no option left.
This is the number of the courier guy
+[protected]

The AWB number is SRTC[protected] and the name of customer is Mehek J