My conclusion is that they are charging Rs 12/- towards each delivery sell transaction. That 12/- is per transaction and no relation with the amount of shares you sold. It's applicable even if you sell a share from your Demat worth only 1/-.
It happened to me, by mistake I took 1 share of Bajaj Hindustan as delivery.
When i sold that one in delivery, I was puzzled to know that they charged Rs 12/- even for this transaction.