Personally, I don't believe such a thing as a voluntary tax exists. A lottery is close but not a true tax since a product is being sold that has some amount of utility for the consumer. Besides, one will find that lotteries generate revenues from the poorest of society. People that are "good" with money realize that the long-term mathematics of that kind of gambling is not in favor of the gambler.
User fees (such as tolls) are very close to a voluntary tax as long as the money is appropriated only to service what is being used. This is where any type of tax approaching voluntary status falls short. Inevitably the money is used for something opposed by the taxpayer (that was not previously announced). If the tax were truly voluntary, the payer should be able to withdraw funds when he or she disagrees with how it is going to be spent, but this is impractical as others would not like bearing a heavier burden when some of the people decided they wanted their money back.
For these reasons (and probably others I haven't thought of yet), I don't believe voluntary taxes can exist in a practical sense. However, having the ability to not pay the tax in the first place would be a mighty check on government. Then, they would find an excuse to make a central banking system so that they could just print up what the taxpayers refused to dole out. Taxpayer meet inflation tax.
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