Actually, it would be a uniform excise tax as allowed by Article I, Section 8, Clause 1 of the U.S. Constiution:
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
You may be correct in your judgement on a National Sales Tax being a Constitutional Excise Tax,however an Excise Tax imposed on ALL goods rather than just on certain goods seems to me to be more over reaching and oppressive a system of taxation than what the Founders intended.
The Government shouldn't need so much money that it has to tax everything.