I suppose I should clarify again--isn't web debate grand?--I am not denying the existence of public goods or collective action problems. (I dispute that all publicly provided goods are genuine collective action problems; there's a great deal of rent-seeking, log-rolling, and sheer stupidity in there.) When I say that your decision to pay, or not, into the treasury voluntarily, is not a collective action problem, I mean that your behavior is not in any way a solution to whatever collective action problems may exist. In the case of a Wal-Mart, it genuinely makes sense to shop there if everyone else does, even if you'd rather not have the Wal-Mart there. I don't think this holds for increasing tax revenue. There is no strategic benefit to doing so in a large polity; it is simply too big for you to have any observable effect on other peoples' committment to taxation.
So while I do understand, quite well, the theory behind which the provision of public goods is a collective action problem, I simply don't think that in practice, as an individual matter, you can decide how much to give to the government/charity on those grounds.