I've been around a long time and a thread I responded to recently triggered a question in my mind. "Am I still using the best routing system for trading"? I've been using ARCA for a very long time.
https://www.elitetrader.com/et/thre...r-scalping-market-making.353519/#post-5272631 Scroll down to see my response.
on hold brothers, is there minimum charge per order? for example, if I send 50 shares to arca, and what the commission would be charged?
You can't send orders for less than 100 shares as a professional trader. I'm not sure what the answer would be if you sent an order for 100 and only got filled for 50. But I'm paying .003 per share, so even if I only get filled on 50, 30 cents isn't going to bother me.
Typically if you cancel the remainder and are stuck with 50 shares you are allowed to sell 50, but you can't open a new ticket to add another 50. You should be able to add 150 though. Either way, you should only get charged for shares traded even if it's partial.