protected function get_tax_class_costs( $order ) {
		$order_item_tax_classes = $order->get_items_tax_classes();
		$costs                  = array_fill_keys( $order_item_tax_classes, 0 );
		$costs['non-taxable']   = 0;

		foreach ( $order->get_items( array( 'line_item', 'fee', 'shipping' ) ) as $item ) {
			if ( 0 > $item->get_total() ) {
				continue;
			}
			if ( 'taxable' !== $item->get_tax_status() ) {
				$costs['non-taxable'] += $item->get_total();
			} elseif ( 'inherit' === $item->get_tax_class() ) {
				$inherit_class            = reset( $order_item_tax_classes );
				$costs[ $inherit_class ] += $item->get_total();
			} else {
				$costs[ $item->get_tax_class() ] += $item->get_total();
			}
		}

		return array_filter( $costs );
	}