"Zero commission" does not actually mean zero commission. The commission just ends up being baked in to the prices they show you. e.g. If the market (institutional) bid/offer for MSFT were 39/41, they would show customers prices of 38/42. For an order of $1000, that would mean they charge you $25 of implicit commission on the trade. It just doesn't show up on your bill. Get real. Their only innovation is hiding fees from their customers, not reducing them.