I have a question

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( ( Item Price - Buyer fee ) - TAX ) - Author fee = Author Earning
Check your Author fee here:

Have you submitted your tax info?

For reducing US backup withholding tax (24%) you should to submit your TAX information form. If you submit the form with your TAX information then only tax will be taken for US sale and TAX percentage depends on your country.
US Tax Treaties:

Otherwise you have to give US backup withholding tax (24%) for all of your sale.