BB of your online platform has a limit price can the online broker charge you more?
They just don't trade your order until it is past your limit price in their favor. If you enter a BUY LMT @ 100 order through an online broker, and the low for the day for that stock is exactly $100, your order will not get traded. But as soon as the stock starts trading at a few cents below $100, say $99.95 or so, then your order to buy it for $100 will get executed. If that was 1000 shares, you just paid $50 more than you should have, but they tell you that you only paid the $6 or whatever scam price they advertise as a commission.
And that's why frequent traders use a broker like interactivebrokers. However, it's not worth it unless you trade a lot. IB is just an order entry platform. So you are responsible for purchasing your own data feed of stock prices and charting software, and that's not cheap.