我通过覆盖运费税过滤器找到了答案

// 0% and 21% tax producdts added combined to the cart needs to have 21% shipping tax
add_filter(\'woocommerce_shipping_packages\', \'override_woocommerce_shipping_packages\');
function override_woocommerce_shipping_packages($packages) {
    $shipment_needs_tax = false;
    foreach ($packages[0][\'contents\'] as $cartitem) {
        if (!empty($cartitem[\'data\']->tax_class)) $shipment_needs_tax = true;
    }

    if ($shipment_needs_tax && empty($packages[0][\'rates\'][\'flat_rate:3\']->get_taxes())) {
        $shipcost = $packages[0][\'rates\'][\'flat_rate:3\']->get_cost();
        $shiptax = $shipcost * 0.21;
        if ($shiptax > 0) $packages[0][\'rates\'][\'flat_rate:3\']->set_taxes([4 => $shiptax]);
    }

    return $packages;
}