Almost all the "commission free" brokerage firms , as they have claimed in their advertisement, have certain execution flaws. Most probably your order will not be filled at desired price as you see on the screen, or you may not even see that your order enters the market if the platform is CFD. The extra bid-ask spread you pay in the long run in fact is a huge lump sum. for example, it is likely that you have to pay a stock $100.20/share in those platform as compared to trade in those DMA market, meaning in average you have to pay 0.2%*2=0.4% for each round trades.
I think you better check first who your counter party is and whether your order is DMA or not. and if it is DMA, usually it will not be commission free.
