GACO: negative tax fee

add_action( 'woocommerce_cart_calculate_fees','new_customers_discount', 10, 1 );
function new_customers_discount( $wc_cart ) {

global $woocommerce;
if (!empty($woocommerce->cart->applied_coupons)) {

    // discount percentage
    $percent = 6;

    // Calculation
    $discount = $wc_cart->cart_contents_total - $wc_cart->get_cart_discount_total * $percent / 100;

    // Add the fee (TAX third argument is disabled: false)
    $wc_cart->add_fee( __( 'Redemption Code Discount', 'woocommerce')." ($percent%)", -$discount, false );
}
}