I'm hardly a "professional" trader, but I've made enough of them to see that when they fill your order with two or more partials, they divide the quoted commission by the relative size of the partial fill until they fill 100% of your order. If the commission quoted to you is $6.99 and it takes 3 partials to fill your order, they don't charge you $20.97. The sum of the commissions for each partial will total $6.99.