public function calculate_shipping( $package = array() ) {
		$rate = array(
			'id'      => $this->id,
			'label'   => $this->title,
			'cost'    => 0,
			'package' => $package,
		);

		// Calculate the costs.
		$has_costs = false; // True when a cost is set. False if all costs are blank strings.
		$cost      = $this->get_option( 'cost' );

		if ( '' !== $cost ) {
			$has_costs    = true;
			$rate['cost'] = $this->evaluate_cost(
				$cost,
				array(
					'qty'  => $this->get_package_item_qty( $package ),
					'cost' => $package['contents_cost'],
				)
			);
		}

		// Add shipping class costs.
		$found_shipping_classes = $this->find_shipping_classes( $package );
		$highest_class_cost     = 0;

		foreach ( $found_shipping_classes as $shipping_class => $products ) {
			// Also handles BW compatibility when slugs were used instead of ids.
			$shipping_class_term = get_term_by( 'slug', $shipping_class, 'product_shipping_class' );
			$class_cost_string   = $shipping_class_term && $shipping_class_term->term_id ? $this->get_option( 'class_cost_' . $shipping_class_term->term_id, $this->get_option( 'class_cost_' . $shipping_class, '' ) ) : $this->get_option( 'no_class_cost', '' );

			if ( '' === $class_cost_string ) {
				continue;
			}

			$has_costs  = true;
			$class_cost = $this->evaluate_cost(
				$class_cost_string,
				array(
					'qty'  => array_sum( wp_list_pluck( $products, 'quantity' ) ),
					'cost' => array_sum( wp_list_pluck( $products, 'line_total' ) ),
				)
			);

			if ( 'class' === $this->type ) {
				$rate['cost'] += $class_cost;
			} else {
				$highest_class_cost = $class_cost > $highest_class_cost ? $class_cost : $highest_class_cost;
			}
		}

		if ( 'order' === $this->type && $highest_class_cost ) {
			$rate['cost'] += $highest_class_cost;
		}

		$rate['package'] = $package;

		// Add the rate.
		if ( $has_costs ) {
			$this->add_rate( $rate );
		}

		/**
		 * Developers can add additional flat rates based on this one via this action since @version 2.4.
		 *
		 * Previously there were (overly complex) options to add additional rates however this was not user.
		 * friendly and goes against what Flat Rate Shipping was originally intended for.
		 *
		 * This example shows how you can add an extra rate based on this flat rate via custom function:
		 *
		 *      add_action( 'woocommerce_flat_rate_shipping_add_rate', 'add_another_custom_flat_rate', 10, 2 );
		 *
		 *      function add_another_custom_flat_rate( $method, $rate ) {
		 *          $new_rate          = $rate;
		 *          $new_rate['id']    .= ':' . 'custom_rate_name'; // Append a custom ID.
		 *          $new_rate['label'] = 'Rushed Shipping'; // Rename to 'Rushed Shipping'.
		 *          $new_rate['cost']  += 2; // Add $2 to the cost.
		 *
		 *          // Add it to WC.
		 *          $method->add_rate( $new_rate );
		 *      }.
		 */
		do_action( 'woocommerce_flat_rate_shipping_add_rate', $this, $rate );
	}