FYI I corrected this problem in a modified invoicing script.
The relevant parts are:

$query = "SELECT DISTINCT invoice_number, billing_id FROM
billing_details
WHERE batch = '$batchid'";
$DB->SetFetchMode(ADODB_FETCH_ASSOC);
$result = $DB->Execute($query) or die ("$l_queryfailed");
$billing_ids = array();
...
while ($myresult = $result->FetchRow()) {
...
$billing_ids[] = $myresult['billing_id'];
}

...
foreach($billing_ids as $billing_id) {
$query = "UPDATE billing SET payment_due_date=DATE_ADD(" .
"(SELECT payment_due_date FROM billing_history WHERE "
.
"billing_id=$billing_id ORDER BY id DESC LIMIT 1), " .
"INTERVAL 1 MONTH) WHERE id=$billing_id";
$DB->Execute($query) or die ("$l_queryfailed '$query'");
}