The General Excise Tax is a cost of doing business and being established in business in Hawaii.
As long as all of the gross receipts, including any collection of this tax that is being passed on to your customers, are being included in the business income these taxes can be listed as an expense on your self employment Schedule C.
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You can choose to include them under "taxes" expense or "miscellaneous" expense. Whatever you decide is the best for you to track your deductions listed.