protected function get_tax_class_costs() {
		$item_tax_classes     = wp_list_pluck( $this->items, 'tax_class' );
		$shipping_tax_classes = wp_list_pluck( $this->shipping, 'tax_class' );
		$fee_tax_classes      = wp_list_pluck( $this->fees, 'tax_class' );
		$costs                = array_fill_keys( $item_tax_classes + $shipping_tax_classes + $fee_tax_classes, 0 );
		$costs['non-taxable'] = 0;

		foreach ( $this->items + $this->fees + $this->shipping as $item ) {
			if ( 0 > $item->total ) {
				continue;
			}
			if ( ! $item->taxable ) {
				$costs['non-taxable'] += $item->total;
			} elseif ( 'inherit' === $item->tax_class ) {
				$costs[ reset( $item_tax_classes ) ] += $item->total;
			} else {
				$costs[ $item->tax_class ] += $item->total;
			}
		}
		return array_filter( $costs );
	}