@Bhuvan
Zerodha charges 20 Rs if we exercise an order. What if we let OTM expire and exercise through the exchange?
Still, zerodha will charge 20 Rs? or I have to pay more because the exchange is participating?
If an option is ITM and gets exercised is when we’re charging brokerage. However, technically we’re allowed to charge brokerage for OTM options also, we aren’t charging brokerage for now.
So technically if we have an OTM sell position so its better to let it expire in order to save brokerage. 
I am assuming zerodha still not charging brokerage for OTM options.
No brokerage if your Option position expires OTM. If the position expires ITM, the brokerage will be applicable.