I have try the optionsxpress as well, my problem is not commission, is the execution. if i setup a credit spread at 0.5 credit in dough, and enter order in optionsxpress, the order couldn't be filled. i found over 50% orders couldn't be filled. if i drop the premium to 50-80% of dough, the orders can be filled. So i change to IB, from paper trading account, i found same problem. Also IB required more capital to trade. For example, for SPX option credit spread -1950/+1945 put 5 contract, IB required $3680 and dough require $2100. I decide change to IB is the deposit is AUD, Optionxpress's account only has USD. i don't want to have another risk from exchange rate.
my mistake for the fund requirement. IB should be same with dough. $2100 to $2500 usd for 5 contract of SPX