an easier way to accomplish this, without the need to add constants, would be to change the formula in invoice.php.
for instance, on my shop i have the last column from the invoice modified from "total including tax" to "tax amount"
to edit, find:

' . $currencies->format(tep_add_tax($order->products[$i]['final_price'], $order->products[$i]['tax']) * $order->products[$i]['qty'], true, $order->info['currency'], $order->info['currency_value']) . '' . "n";

change to:

' . $currencies->format($order->products[$i]['tax'] * $order->products[$i]['qty'], true, $order->info['currency'], $order->info['currency_value']) . '' . "n";

More...