I've never tried canceling the remainder, but yes you would be charged commission for the total amount traded (with the $2.50 minimum), I presume if one modified the order after a partial fill, such as changing the limit price, it would probably count as a new order and a separate minimum would apply if/when that was filled. Yes, I found the partial fills to be infrequent with that level of order sizes, the last one I encountered was on a Sunday evening. If you were to trade the minimums (whatever that is, 25 or 30K, I think) I presume partials could not happen, but then your commission as a percentage of the trade size would be higher.