add this shortcode and modify it to yourtheme/functions.php or snippets code plugin

add_filter('op_cashdrawer_login_session_data',function($session_data){
    
   
    $session_data['setting']['custom_fees'] = array(
        array('title' => 'sample fee 5 percentage', 'amount' => 5, 'type'=> 'percent'),
        array('title' => 'sample fee 7 fixed ', 'amount' => 7, 'type'=> 'fixed'),
       
    );
    
    return $session_data;
},30,1);

when add fee , a quick custom amount display