Greedy Wisconsin

The Greedy State of Wisconsin and other states have been collecting sales tax on postage for several years now. I can understand that when paying for the shipment of a purchased item by mail the handling fees like any other service would be taxable but not the actual postage charged by the United States Post Office which is still part of the Federal Government. No state has the legal right to tax the Federal Government. If you think I am wrong in this view, ask yourself why is it that when you go to the post office and ship a parcel or buy some stamps the post office does not charge sales tax on these transactions. Granted it is easier to charge the sales tax on the entire amount rather than to separate the taxable handling charges from the actual postage but it still is wrong to do so. Since the amount of money per transaction or per individual is too small to justify a privately funded law suit no one has yet challenged this illegal taxation. Maybe it is time to consider a class action suit? Any lawyers out there? If so please feel free to chime in.