Example of how to trigger messages to send when a user 'revisits' spco and updates their registration.

tw_ee_trigger_messages_on_revisit.php

<?php // Please doe no include the opening PHP tag if you already have one.

add_action(
     'AHEE__EE_Single_Page_Checkout__process_attendee_information__end',
     'tw_ee_trigger_messages_on_revisit',
     10,
     2
);
function tw_ee_trigger_messages_on_revisit($spco, $valid_data) {
    if ($spco->checkout->revisit) {
        $transaction = $spco->checkout->transaction;
        
        // No transaction? Get outta here.
        if(! $transaction instanceof EE_Transaction ) {
            return
        }

        $trigger_notifications = true;
        // now update the registrations and add the results to our $update_params
        if ($trigger_notifications) {
            // Add filter to send messages.
            add_filter('FHEE__EED_Messages___maybe_registration__deliver_notifications', '__return_true', 15);
            
            // The function used to sned message checks for values on the request, include those to force it to send.
            $update_params = array(
                'finalized'       => true,
                'revisit'         => $spco->checkout->revisit,
                'status_updates'  => true,
            );            
            // send messages
            /** @type EE_Registration_Processor $registration_processor */
            $registration_processor = EE_Registry::instance()->load_class('Registration_Processor');
            $registration_processor->trigger_registration_update_notifications(
                $transaction->primary_registration(),
                $update_params
            );
        }
    }

}