It is probably an issue when one or more of the fields is blank. Maybe add some IF statements to correct that like:
//Attachment Entries
if ($Payments->getColumnVal("ATOLCert")) $WA_MailObject = WAUE_AddAttachment($WA_MailObject,"clientconf/".($Payments->getColumnVal("ATOLCert")) ."");
do {
if ($row_service && $row_service["UploadConf"]) $WA_MailObject = WAUE_AddAttachment($WA_MailObject,"clientconf/".$row_service["UploadConf"] ."");
} while ($row_service = mysql_fetch_assoc($service));
if ($ServiceFlight->getColumnVal("UploadConf")) $WA_MailObject = WAUE_AddAttachment($WA_MailObject,"clientconf/".($ServiceFlight->getColumnVal("UploadConf")) ."");