Some broker don't allow client direct their orders to a specific trading venue, normally they charge $3 to $5 per trade regardless the quantities. Some offer direct access and charge $0.0035 per share,client can get rebate normally $0.002/share for adding liquidity if order is displayed on ecn book ,or pay $0.003/share for taking liquidity if using mkt order. $3 to $5 per trade commission structure seems good for trading large quantities using mkt order, but will broker send my order to a counterparty with larger spread just to get larger payments for order flow? Why IB don't offer $3 to $5 per trade commission structure for stocks?