WooCommerce Allow Shipping Countries: Make orders to be shipped only in specific country, display message if the customer don't choose an allowed one.

functions.php

<?php
/**
 * WOOCOMMERCE ALLOW SHIPPING COUNTRIES
 * Make orders to be shipped only in specific country, display message if the customer don't choose an allowed one.
 */
function custom_checkout_process() {
    global $woocommerce;

    // Block orders to be shipped to other country than New Zealand, United States and Australia
    $allowed_shipping_countries = array( 'AU', 'NZ', 'US' );
    $error_message = 'We only deliver to Australia, New Zealand and United States';

    if ( isset( $_POST['shiptobilling'] ) && $_POST['shiptobilling'] == '1' ) {
        if ( ! in_array( $_POST['billing_country'], $allowed_shipping_countries ) )
            $woocommerce->add_error( $error_message );
    } else {
        if ( ! in_array( $_POST['shipping_country'], $allowed_shipping_countries ) )
            $woocommerce->add_error( $error_message );
    }
}
add_action('woocommerce_checkout_process', 'custom_checkout_process');