3x5 notecard. Gross income x 0.07. Submit to US Treasury Everyone pays. No deductions.
What about people who get income from various sources such as rental income, business income, stocks, bonds, etc.?
Maybe the final calculation will fit on a single 3x5 card, but there are so many ways to generate income that it could be pages and pages to list for some folks.
And for those who do make money on something like real estate: do you really want them to just state how much money they charged for rent without being able to specify all of their deductions for repairs, property taxes, etc.?
The flat tax, like the Non-Aggression Policy, like libertarianism in general, sounds great at first but is just childish fantasy.