you don't get charged extra when the MM's break an order up to fill it. Your BROKERAGE charges you a fee for ONE purchase/Sale. MM's don't necessarily get the shares you want at one place, or at one price. They will get the cheapest ones first (most profit for them), then get some more at a higher price. With AON, you must wait until they find all the shares at any price (at their bid or less) or you don't get any. I never use AON - I set a limit and let them fill over time. They have 60 days to fill it, and almost always do fill.
MM's don't charge YOU a fee at all. You really need to learn how the system works if you don't want to lose your money.