If you are more concern about the amount of the fee rather the reliability of your ticket to be fill without slippage, you could choose Ameritrade (Datek) for a fix fee of 9.95 by ticket or 20$ for your roundtrip.
check out www.betatrading.net They have max ticket where you can't get charged more than the capped price. it is being offered at $11.99 regardless a number of shares.