A National Flat Tax on income/gains, administered by the States, who collect and send the money on to Mordor.
Even if you go with a national sales tax, there's no reason it can't be administered the same way. States comptrollers already collect sales tax from businesses. The sales tax report is literally the size of a post card - Gross sales less exempt sales = taxable sales X the state rate. Sign it and take a tiny discount for early filing (Texas).