Zero-commission does not mean zero cost. It usually means the revenue has moved to payment-for-order-flow, wider effective spreads, margin interest or securities lending — costs that are real but invisible on the confirmation.
Compare schedules on total cost per round trip at your actual size, including exchange-fees, regulatory-fees and any data charges. Per-share plans favour high-priced stocks; flat plans favour large share counts.
Example: 400 round trips a month of 500 shares. A $0.005 per share plan with a $1 minimum costs 400 x 2 x $2.50 = $2,000. A flat $4.95 per trade costs $3,960. A zero-commission broker costs nothing visible but delivers $0.0015 a share less price-improvement — about $600 a month, hidden inside the fills.
Related: exchange-fee, regulatory-fee, payment-for-order-flow, price-improvement