I can now tell you that their advertised brokerage rates are for trades that settle with a contract for each trade. So up to $5000 trade, the brokerage is $5
If you choose end of day settlement, then trades in the same stock on the same day are aggregated into the one contract. This works ok for trades that total over $10k on that day. However, lets say someone takes a nibble at your order, and the trade in that stock only totals $10 for the day, you will get charged $15 brokerage if your preference is set to end of day settlement
Seeing as the rates for " end of day settlement" are different to the advertised rates, I asked for the schedule of these rates several times, but they wouldnt disclose them.
