Hi there.
Index Option is cash-settled. You get nothing if the position is expired OTM (since you have received the full premium when you shorted). If expires ITM, you pay the premium.
Charges? No brokerage charged if expires OTM. If your position expires ITM, the brokerage will be applicable. And there’s no STT since it’s only applicable on sell side which you have paid.
For more info, refer to the thread here: What happens if I don't square off my positions in options? - #2 by ShubhS9