When I add in my office expenses and supplies for business, my return gets lower. How come?

By increasing your deductions and expenses it will decease your income and you may not be getting as many credits as before like the CWB (Canada workers Benefit) credit. You can't go by the refund monitor until you have entered everything in. Also after you reduce your income to zero there is no more refund to get back. But even though your taxable income goes down and you owe less income tax on it, you still owe SE Self Employment tax if you have a Net Profit.

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