In includes/classes/order.php look for the line saying

EMAIL_TEXT_INVOICE_URL . ' ' . zen_href_link(FILENAME_ACCOUNT_HISTORY_INFO, 'order_id=' . $zf_insert_id, 'SSL', false) . "\n\n";

it's on line 1069 in 1.5.7c. Just adjust that to point to FILENAME_INVOICE instead.