Thanks @TheDawn. Fees on spy/spx 231P flys add up so I'll need to confirm with them. Their options fees page mentions: Premium < USD 0.05 ... USD 0.25/contract, plus the exchange etc fees https://www.interactivebrokers.com/en/pricing/commissions-options.php
They might have changed it. IBKR used to own an option exchange and then they sold it citing conflict of interest. So things might have changed since then.