The flat tax, sales tax and VAT are all border neutral.
With the caveat that no special credit is required in the retail sales tax case to refund taxes paid on goods manufactured, as no retail tax exists for business to business purchases. No red tape, far less in the way of regulatory cost to manufacturers and exporters.
Flat Tax, don't see how you are going to make that one fly, as it is both a wage tax coupled with a corporate income tax. Just removing taxes on investment/savings income per-se does not a border neutral/adjustable tax make.
With at VAT applying the credit back to the exporter does the trick, under a flat tax no practical way to accomplish that for the full chain of taxable levels of production that goes into an export product. Flat Tax doesn'et have the mechanism built in for crediting all stages of tax fowarding.
No caveat required. No credits. No forms. No nothing. All VATs are border neutral including the flat tax and sales tax.