That doesn't even make sense in neoclassical economics. One person's effects on the market are necessarily tiny, but if buying more of Brand X has any effect it

That doesn't even make sense in neoclassical economics. One person's effects on the market are necessarily tiny, but if buying more of Brand X has any effect it will be to drive up the price of Brand X. You're actually harming "those really in need", and you should feel bad about it.