This is a bad business. Competing on cost against large incumbents without a defensible low cost position in a competitive industry without most target users paying high costs currently (interactivebrokers).
You must pay at least $10 in commissions each month or they charge you the difference. Depending on how much you trade this might be a deal breaker. There is also a minimum initial deposit.
Virtualbrokers is another option with more choice of fee structure. I don't think they have a minimum monthly fee but they charge you for data.