What hidden fees should I watch for with trading platforms?

Beyond commissions: payment for order flow reduces execution quality ($0.01-0.03 per share on Robinhood), options contract fees ($0.50-0.65 each), margin interest (6-13% APR), wire transfer fees ($25-75), account transfer fees ($50-100), and inactivity fees on some platforms. Fractional share spreads are also wider than full shares. Always check the fee schedule before opening an account.