A national sales tax would require each retail outlet to account for the tax. A flat tax would be accounted with the employer which is already being done though witholding tax. There is no impact on the retail market and no impact on the employer.
Retail outlets already account for state sales taxes, including the fact that many states have multi-tiered tax schemes (food taxed at one rate, etc.).
It sure hasn’t stopped states from collecting taxes.
I think modern computer software can handle this task.
Further, at least under 999, there would be NO withholding tax for the employer to account for.