Let's say I execute a forex order for 30,000 units though IB, let's say I only get filled for 20,000 units, a few of questions come up... 1. Do I pay commission on the 30K or the 20K? 2. Since IB's commissions go up for orders under 25K does this them mean I would pay the high commission trying to fill the remaining 10K? 3. How do people handle partial fills, is there some threshold, say getting filled for 80% of the order, where people don't take any action? Any help appreciated.