They only offer Market, Limit, Stop, and Stop Limit orders. I don't need market-on-open orders as NYSE market orders submitted prior to the open are supposed to fill at the open, due to the auction process. Zecco routes all orders through Penson. I have never filled out a W-9 and they do not withhold taxes for me.
Do they pay interest on balances? If not then there's your 'hidden cost' - they probably make 5% on all deposits. Even IB doesn't pay interest on the first 10k balance so that's like an easy 500 dollars they make a year (hidden cost) for each account.
aeliodon, Zecco's web site says they do pay. Besides, there are other avenues for the people to earn money rather than just sit on cash for interests in a trading account.