Supplies and all expenses should be subtracted from the income to give you a Net Profit or Loss on Schedule C. Why do you think it's not? Yes your taxable income should go down. But you know you are paying 2 kinds of tax on Schedule C self employment income. You pay regular income tax on all your taxable income and self employment tax on the Net Profit on Schedule C.
Where are you entering the income and expenses? Maybe you are not entering the supplies on Schedule C?
