I also had IDFC Infra bonds in physical form. When I had enquired for dematerialization, I was told Zerodha would charge Rs 150 per certificate + Rs 100 courier charges+18% GST.
I went to HDFC Demat desk and got it dematerialized for only Rs 45 in my HDFC Securities account.
What I have noticed is Zerodha while equity delivery is free and intraday trading charges are less compared to other brokers, the other services are quite expensive. More than the money it is a time-consuming process. Imagine you sending documents by courier. Them receiving it, and then they send it to the RTA for the dematerialization. How much time it takes. Too long shot.