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.
Especially when you consider the fairtax includes taxing services, which is really a tax directly on the fruits of ones labor. Hardly something a Judge in 1800's would have gone along with as Constitutional.