Custom shipping method for WooCommerce (minimal boilerplate required)

main.php

<?php

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly.
}

// Hooks
add_action( 'plugins_loaded', 'yourprefix_include_shipping_method' );
add_action( 'woocommerce_shipping_methods', 'yourprefix_register_shipping_method' );

function yourprefix_include_shipping_method () {
	include 'shipping-method.php';
}

function yourprefix_register_shipping_method ( $methods ) {
	$methods['your-shipping-method-id'] = 'YourPrefix_Shipping_Method'; // full class name
	return $methods;
}

shipping-method.php

<?php

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly.
}

class YourPrefix_Shipping_Method extends \WC_Shipping_Method {
	public $id;
	public $instance_id;
	public $method_title;
	public $method_description;
	public $supports;

	public $enabled;
	public $title;

	public function __construct( $instance_id = 0 ) {
		$this->id = 'your-shipping-method-id';
		$this->instance_id = absint( $instance_id );
		$this->method_title = __( 'Your Shipping Method Name', 'textdomain' );
		$this->method_description = __( 'Your Shipping Method Description', 'textdomain' );
		$this->supports = array(
			'shipping-zones',
			'instance-settings',
			'instance-settings-modal',
		);
		$this->init();
	}

	function init() {
		// Load the settings
		$this->init_form_fields();

		// Define user set variables
		$this->title = $this->get_option( 'title' );

		// Actions
		add_action( 'woocommerce_update_options_shipping_' . $this->id, array( $this, 'process_admin_options' ) );
	}

	function init_form_fields() {
		$this->instance_form_fields = array(
			'title' => array(
				'title' => __( 'Title', 'textdomain' ),
				'type' => 'text',
				'description' => __( 'This controls the title which the user sees during checkout.', 'textdomain' ),
				'desc_tip' => true,
				'default' => $this->method_title,
			),
			
			/**
			 * Define your other settings
			 */
		);
	}

	public function calculate_shipping( $package = array() ) {
		// Check if valid to be calculeted.
		if ( '' === $package['destination']['postcode'] ) {
			return;
		}
		
		// Do your shipping logic (e.g. request an API)

		$this->add_rate(
			array(
				'label' => $this->title,
				'cost' => 99,
				'taxes' => false,
			)
		);
	}
}