I figured this out myself, woohoo! Crude, but it works.
in admin/invoice.php line 64
I inserted:
<?php
$order_check = $db->Execute("select ... account_name, account_number,
po_number, ... from " . TABLE_ORDERS . "
where orders_id = '" . (int)$oID . "'");
and farther down, around line 132, I added a column to the embedded table,
<table border="0" cellspacing="0" cellpadding="2">
<tr>
<td class="main"><strong><?php echo ENTRY_DATE_PURCHASED;
?></strong></td>
<td class="main"><?php echo zen_date_long($order->info['date_purchased']); ?></td>
<td class="main"> </td>
</tr>
<tr>
<td class="main"><b><?php echo ENTRY_PAYMENT_METHOD; ?></b></td>
<td class="main"><?php echo $order->info['payment_method']; ?></td>
<td class="main">
<?php
// purchaseorder start
if( (($order->info['account_name']) || ($order->info['account_number']) ||
($order->info['po_number'])) ) {
?>
<div><?php echo 'Account: ' . $order->info['account_name']; ?></div>
<div><?php echo 'Account number: ' . $order->info['account_number']; ?></div>
<div><?php echo 'PO number: ' . $order->info['po_number']; ?></div>
<?php
// purchaseorder end
}?>
</td></tr>
</table>
I will apply the equivalent modification to packingslip.php.
Perhaps this info will help someone else.
The one thing I still don't know, where are customer comments saved in the database (if anywhere) and is there a way to display them on the invoice and packing slip.
---Diana (now an intermediate zen user?)