TurboTax does deduct line 10c from AGI now. Are you looking at the actual Form 1040? Does the contribution amount appear on line 10b?
If you entered the contribution under Charitable Donations in the Deductions & Credits section, TurboTax doesn't make the entry on Form 1040 until you finish that section. After you click "Done with Deductions," a few screens later it tells you that you are getting the standard deduction. When you click Continue on that screen, the next screen tells you that it has added the $300 contribution to your tax return. Only at that point is it deducted from your AGI.
