If a flat-tax was used why would a postcard be necessary?

Not a lot of details about how this works over the decades of the idea being tossed around I am still baffled.

#1: The government straight up puts it on the employer to pay your tax and you shouldn’t have to deal with anything.

#2: Some postcard shit where you pay at the end of the year. It should be pretty clear why that wont work. People suck at saving.

Here is the Ted Cruz postcard:

https://www.tedcruz.org/wp-content/uploads/2015/10/Graphic3.png

To be honest that looks just as complicated as my current taxes. Enter itemized deductions without any explanation of what that means.. That is why you fit the shit on a postcard and my 1040EZ is 25 pages.

https://www.tedcruz.org/tax_plan/

Am I missing it or does he say if he wants the employer to collect or does he expect the employee to save and hope they can pay at the end of the year?