Under $500 trades costing me hard in brokerage fees, page-3

BTW, in the example you used where only 46k of a 500k order traded, as long as you don't cancel the remaining order it should execute under the same brokerage charge - you should be able to modify the order to a different price without incurring an extra brokerage charge. If you cancel then you have to pay full brokerage on the partial trade.