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In reply to comment: not sure how true this is (see in situ)

What they meant when they said it doesnt matter is that 'they' (as in 'not you') get the three dollars either way. You dont get it back. But you can put it into the general fund, or into the Presidential Campaign Fund instead. Your choice.

So if you check the box you voluntarily donated it, if you dont then 'they' (meaning the government, not the Presidential campaigns) take it involuntarily, either way you lose it.