protected function get_item_costs_by_tax_class() {
		$tax_classes = array(
			'non-taxable' => 0,
		);

		foreach ( $this->items + $this->fees + $this->shipping as $item ) {
			if ( ! isset( $tax_classes[ $item->tax_class ] ) ) {
				$tax_classes[ $item->tax_class ] = 0;
			}

			if ( $item->taxable ) {
				$tax_classes[ $item->tax_class ] += $item->total;
			} else {
				$tax_classes['non-taxable'] += $item->total;
			}
		}

		return $tax_classes;
	}