If the extra cost of the taxes is what's keeping you from donating, perhaps you can send in a reduced amount instead then? Foe example, if you wanted to donate $100 and your overall tax rate is 40% (resulting in an overall reduction of $40 from your taxable income), you could instead donate $60 and save the other $40 for your pending tax bill.

(Just an example... I'm not an accountant and I live in the US so I don't know if things are different in Canada, etc etc)