get_post_meta not showing the value of custom field when called - wordpress

Question

I've had the following code to add a custom order field in the woocommerce checkout:
function delivery_time_slots_array () {
return array(
'' => _('Choose a time slot'),
'5pm - 6pm' => __('5pm - 6pm', 'woocommerce'),
'6pm - 7pm' => __('6pm - 7pm', 'woocommerce'),
'7pm - 8pm' => __('7pm - 8pm', 'woocommerce'),
'8pm - 9pm' => __('8pm - 9pm', 'woocommerce')
);
}
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
$fields['order']['order_delivery_time'] = array(
'type' => 'select',
'label' => __('Delivery time (choose an hour slot between 5pm & 9pm)', 'woocommerce'),
'class' => array('form-row-wide'),
'required' => true,
'options' => delivery_time_slots_array()
);
Then, followed by the below code to update the order meta with field value:
add_action( 'woocommerce_checkout_update_order_meta', 'custom_checkout_field_update_order_meta', 10, 2 );
function custom_checkout_field_update_order_meta( $order_id ) {
if ( ! empty( $_POST['order_delivery_time'] ) ) {
update_post_meta( $order_id, 'Delivery time', sanitize_text_field($_POST['order_delivery_time']) );
}
}
This all displays correctly and returns the value within the Orders section of woocommerce. However, I can't then access this data to return/display it on the thank-you page. I'm using the following code at present:
<li class="delivery_slot">
<?php _e( 'Delivery Slot', 'woocommerce' ); ?>
<strong><?php
$delivery_slots = delivery_time_slots_array();
$delivery_slot = get_post_meta($order_id, 'Delivery time', true);
if( isset($delivery_slots[$delivery_slot]) )
echo $delivery_slots[$delivery_slot];
?></strong>
I've reviewed 2 hours of posts here & on Google including the below example (which my code mirrors), but cannot get the value to display:
How to get the value instead of order_id with get_post_meta()

Answer 1

If you attach it to any of the hooks in the order-details.php template then the $order object will be available in your callback function
function so_34215698_display_order_meta( $order ){
$delivery_slots = delivery_time_slots_array()
$delivery_slot = get_post_meta($order->id, 'Delivery time', true);
if( isset($delivery_slots[$delivery_slot]) )
echo $delivery_slots[$delivery_slot];
}
add_action( 'woocommerce_order_details_after_order_table', 'so_34215698_display_order_meta' );