So if I write a stand alone app and send directed orders via API, IB charges an extra fee vs. sending directly from TWS. I assume that when your customers send such orders, they do not incur this extra fee because IB knows that the account is associated with some third party platform (like DAS trader). I don't know this for certain, but it would be a major deterrent if this was the case.
@MotiveWave, I assume you guys have IB Gateway running which serves all your customers so IB knows that the orders were manually submitted via your GUI(not sure how they treat SDK orders).