@varnco:
I managed to get the order/invoice number in the E-mail subject using HTML-mail:
edit "email_invoice.php" in catalog/includes/modules/email_invoice
1. find this code (around line 107):
$ei_message->send($order->customer['name'], $order->customer['email_address'], STORE_OWNER, STORE_OWNER_EMAIL_ADDRESS, EMAIL_TEXT_SUBJECT);
and replace this with:
$new_mail_subject = EMAIL_TEXT_SUBJECT . ' - ' . EMAIL_TEXT_ORDER_NUMBER . ' ' . $insert_id;
$ei_message->send($order->customer['name'], $order->customer['email_address'], STORE_OWNER, STORE_OWNER_EMAIL_ADDRESS, $new_mail_subject, $email_order);
2. then find this code (around line 110):
$ei_message->send(STORE_OWNER, SEND_EXTRA_ORDER_EMAILS_TO, $order->customer['name'], $order->customer['email_address'], EMAIL_TEXT_SUBJECT);
and replace this with:
$ei_message->send(STORE_OWNER, SEND_EXTRA_ORDER_EMAILS_TO, $order->customer['name'], $order->customer['email_address'], $new_mail_subject, $email_order);
Hope this will help you.
BTW: The original code for adding the order# in the E-mail subject is found in this contrib: http://www.oscommerce.com/community/contributions,2878