Thank you for your question.
Commission expenses are claimed as employment expenses; you will claim these expenses on Form T777 - Statement of Employment Expenses. To do this on TurboTax, follow these instructions:
1) Click on "Find" on the top right of your screen and type in "employment expenses".
2) Clarify whether you received a form T2200 or T2200S from your employer.
3) Where it asks "Do you want to limit expenses to your commission income (if applicable)?" check off yes (for a commissioned employee, your expenses cannot exceed the total of your commissioned income).
4) Put in your commission expenses on the following page.
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