Send hourly email to the users who don't pay the total yet.

Send Email To All Unpaid Users FF

//Send Email To unpaid/pending users via cron job wp (FF payment status = pending)
/**
 * this method will register event to WordPress init
 */
add_action( 'init', 'sendEmailToPaymentPendingUsers');
function sendEmailToPaymentPendingUsers(){
    if( !wp_next_scheduled( 'notify_unpaid_users' ) ) {
        // schedule an event
        
        //Available Event Types (hourly, twicedaily, daily, weekly, fifteendays, monthly)
    
        wp_schedule_event( time(), 'hourly', 'notify_unpaid_users' );
    }
}

add_action( 'notify_unpaid_users', 'sendEmailToUnpaidUsers' );

/**
 * this method will call when cron executes
 */
function sendEmailToUnpaidUsers() {
    /**
        *$formId = 14 please replace the 14 with your form ID
     **/
     
    $formApi = fluentFormApi('forms')->entryInstance($formId = 14);
    $entries = $formApi->entries();
    $count = count($entries);
    $subject = 'Please Pay';
    $message = 'Hi, <br> Please pay the total.';
    $send_email_to = array();
    for ($i=0; $i<$count; $i++) {
        if($entries['data'][$i]->payment_status === 'pending'){
            $send_email_to[] =$entries['data'][$i]->response['email'];
        }
    }

    wp_mail($send_email_to, $subject, $message);
}