Example of how to alter the MER cart model success message.

example.php

<?php

//Alter the successfully added for this event string
function tw_ee_change_cart_result_message($original_text, $ticket_count) {
  $cart = EE_Registry::instance()->SSN->cart();

  if( $cart instanceof EE_Cart ) {
    $output = array();
    $tickets_array = $cart->get_tickets();
    foreach( $tickets_array as $ticket) {
        if($ticket instanceof EE_Line_Item) {
            $output[] = $ticket->get('LIN_name');
        }
    }
    $list = implode(', ', $output); 
    return 'You have ticket(s):<br>' . $list . '<br>in the cart.';
  }
    return $original_text;
}
add_filter('FHEE__EED_Multi_Event_Registration__get_cart_results_results_message', 'tw_ee_change_cart_result_message', 10, 2);