Not paying taxes is a different issue. The question is a) The obligation of Dina D’Malchusa, which the poskim seem to apply to the United States (Israel, of course, is a different can of worms). But the question is then what is noheg. If the entire USA does not pay taxes, or at least a significant proportion don’t, then the law is not really enforced. We deal with realities. For instance, jaywalking when there is no car in sight presumably would not a violation of Dina D’Malchusa.
But, I personally think that the goverment is makpid on cash sales, certainly for large items when the sum is significant. Has there ever been real enforcement in this area? Or is it in the empty realm of unenforced laws and uncollected fines?