[Shipping Method API] WooCommerce skeleton shipping method plugin code example.

wc-skeleton-shipping-method-example.php

<?php
/*
Plugin Name: Your Shipping plugin
Plugin URI: https://woocommerce.com/
Description: Your shipping method plugin
Version: 1.0.0
Author: WooThemes
Author URI: https://woocommerce.com/
*/

/**
 * Check if WooCommerce is active
 */
if ( in_array( 'woocommerce/woocommerce.php', apply_filters( 'active_plugins', get_option( 'active_plugins' ) ) ) ) {

	function your_shipping_method_init() {
		if ( ! class_exists( 'WC_Your_Shipping_Method' ) ) {
			class WC_Your_Shipping_Method extends WC_Shipping_Method {
				/**
				 * Constructor for your shipping class
				 *
				 * @access public
				 * @return void
				 */
				public function __construct() {
					$this->id                 = 'your_shipping_method'; // Id for your shipping method. Should be uunique.
					$this->method_title       = __( 'Your Shipping Method' );  // Title shown in admin
					$this->method_description = __( 'Description of your shipping method' ); // Description shown in admin

					$this->enabled            = "yes"; // This can be added as an setting but for this example its forced enabled
					$this->title              = "My Shipping Method"; // This can be added as an setting but for this example its forced.

					$this->init();
				}

				/**
				 * Init your settings
				 *
				 * @access public
				 * @return void
				 */
				function init() {
					// Load the settings API
					$this->init_form_fields(); // This is part of the settings API. Override the method to add your own settings
					$this->init_settings(); // This is part of the settings API. Loads settings you previously init.

					// Save settings in admin if you have any defined
					add_action( 'woocommerce_update_options_shipping_' . $this->id, array( $this, 'process_admin_options' ) );
				}

				/**
				 * calculate_shipping function.
				 *
				 * @access public
				 * @param array $package
				 * @return void
				 */
				public function calculate_shipping( $package = array() ) {
					$rate = array(
						'label' => $this->title,
						'cost' => '10.99',
						'calc_tax' => 'per_item'
					);

					// Register the rate
					$this->add_rate( $rate );
				}
			}
		}
	}

	add_action( 'woocommerce_shipping_init', 'your_shipping_method_init' );

	function add_your_shipping_method( $methods ) {
		$methods['your_shipping_method'] = 'WC_Your_Shipping_Method';
		return $methods;
	}

	add_filter( 'woocommerce_shipping_methods', 'add_your_shipping_method' );
}

BUG: the value of $rate is not actually a valid argument for the add_rate method here:

$rate = array(
	'label' => $this->title,
	'cost' => '10.99',
	'calc_tax' => 'per_item'
);

// Register the rate
$this->add_rate( $rate );

In the comments for the add_rate method, it says that 'cost' is either a single amount or an array, and 'calc_tax' is either 'per_order' or 'per_item'. But when 'per_item' is used, then 'cost' needs to be an array. If you use 'per_item' with a single amount for 'cost', the result will be that the calculated tax is always zero.