public function get_extra_cost( $cost_string, $type, $package ) {
		$cost         = $cost_string;
		$cost_percent = false;
		// @codingStandardsIgnoreStart
		$pattern      =
			'/' .           // Start regex.
			'(\d+\.?\d*)' . // Capture digits, optionally capture a `.` and more digits.
			'\s*' .         // Match whitespace.
			'(\+|-)' .      // Capture the operand.
			'\s*' .         // Match whitespace.
			'(\d+\.?\d*)' . // Capture digits, optionally capture a `.` and more digits.
			'\%/';          // Match the percent sign & end regex.
		// @codingStandardsIgnoreEnd
		if ( preg_match( $pattern, $cost_string, $this_cost_matches ) ) {
			$cost_operator = $this_cost_matches[2];
			$cost_percent  = $this_cost_matches[3] / 100;
			$cost          = $this_cost_matches[1];
		}
		switch ( $type ) {
			case 'class':
				$cost = $cost * count( $this->find_shipping_classes( $package ) );
				break;
			case 'item':
				$cost = $cost * $this->get_package_item_qty( $package );
				break;
		}
		if ( $cost_percent ) {
			switch ( $type ) {
				case 'class':
					$shipping_classes = $this->find_shipping_classes( $package );
					foreach ( $shipping_classes as $shipping_class => $items ) {
						foreach ( $items as $item_id => $values ) {
							$cost = $this->calc_percentage_adjustment( $cost, $cost_percent, $cost_operator, $values['line_total'] );
						}
					}
					break;
				case 'item':
					foreach ( $package['contents'] as $item_id => $values ) {
						if ( $values['data']->needs_shipping() ) {
							$cost = $this->calc_percentage_adjustment( $cost, $cost_percent, $cost_operator, $values['line_total'] );
						}
					}
					break;
				case 'order':
					$cost = $this->calc_percentage_adjustment( $cost, $cost_percent, $cost_operator, $package['contents_cost'] );
					break;
			}
		}
		return $cost;
	}