You already knew that we don't have a state sales tax, right? That 4.166% extra that you pay is not sales tax but general excise tax. The difference is that a sales tax is a tax on the buyer but the GET is a tax on the seller.
Every time goods change hands in Hawaii, the GET is charged. There are no exceptions here as there are in other states. Groceries, prescriptions, milk and even doctor's visits are subject to Hawaii's GET.
Please see this article from January 3rd's Star-Bulletin for more information about how this tax works.