How much are your Itemized Deductions? The questions I've seen turn out that the Standard Deduction + the extra 300 for charity is more than your Itemized Deductions . Check 1040 line 10b.
And by increasing your deductions it will decease your income and you may not be getting as many credits as before. Also after you reduce your income to zero there is no more refund to get back but you still may owe for other things like self employment tax or the 10% early withdrawal penalty from 401k or IRA accounts.
