If you get a 1099, they are not your employer and you are not an employee. You are self-employed, in business for yourself. The money that you earn is business income.
Does the 1099 show the net amount that you receive after subtracting the commission, or does it show the total amount that the customer pays, before subtracting the commission?
If the 1099 shows the net amount that you receive, then the commission is already written off. You can't write it off again. You just report the amount of income shown on the 1099.
If the 1099 shows the total amount, before subtracting the commission, then you can deduct the commission as a business expense, under commissions.
