[Customizing checkout fields using actions and filters] Add custom WooCommerce checkout field to emails

wc-custom-checkout-field-display-in-emails.php.php

/* To use: 
1. Add this snippet to your theme's functions.php file
2. Change the meta key names in the snippet
3. Create a custom field in the order post - e.g. key = "Tracking Code" value = abcdefg
4. When next updating the status, or during any other event which emails the user, they will see this field in their email
*/
add_filter('woocommerce_email_order_meta_keys', 'my_custom_order_meta_keys');

function my_custom_order_meta_keys( $keys ) {
     $keys[] = 'Tracking Code'; // This will look for a custom field called 'Tracking Code' and add it to emails
     return $keys;
}

Full code for adding custom field by product ID, then displaying field value on the order edit page, and then adding to the emails.

* Add fields to the checkout page based on products in cart.
 *
 * @how-to        https://remicorson.com/?p=7871
 * @author        Remi Corson
 * @testedwith    WooCommerce 3.4.0
 */

add_action( 'woocommerce_checkout_fields', 'woo_add_conditional_checkout_fields' );

function woo_add_conditional_checkout_fields( $fields ) {

	foreach( WC()->cart->get_cart() as $cart_item ){
	    $product_id = $cart_item['product_id'];

			if( $product_id == 947 ) {
				$fields['billing']['billing_X'] = array(
					'label'     => __('Billing X, 'woocommerce'),
					'placeholder'   => _x('', 'placeholder', 'woocommerce'),
					'required'  => true,
					'class'     => array('form-row-wide'),
					'clear'     => true
				);
			}

	
	}

	// Return checkout fields.
	return $fields;

} 

/**
 * Display field value on the order edit page
 */
 
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );

function my_custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('Billing X').':</strong> ' . get_post_meta( $order->get_id(), '_billing_X', true ) . '</p>';
}

/**
 * Add a custom field (in an order) to the emails
 */
add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );

function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['billing_X'] = array(
        'label' => __( 'Billling X' ),
        'value' => get_post_meta( $order->id, '_billing_X', true ),
    );
    return $fields;
}