Zerodha Options Buy - first cash will be used for option buys and once there is no free cash then only collateral is used for option buying.
When We use collateral margin for option buying, Delayed Payment Charges (DPC) of 0.05% per day (₹50 per lakh) or 18% per annum apply only to overnight positions. DPC is charged only on the collateral amount utilised for option buying, not the entire option purchase amount.
refer article : Can I buy options using collateral margin?