code changes to add invoice number to the cc receipt starting at about line 239  function process_button
    function process_button() {
      global $order,$FSESSION,$FREQUEST;
//add hjmsoftware to add order number
$last_order_id = tep_db_query("select * from ".TABLE_ORDERS." order by orders_id desc limit 1");
$last_inv = tep_db_fetch_array($last_order_id);
$new_order_id = $last_inv['orders_id'] + 1;

// end code		
	  
      $sequence = rand(1, 1000);
      $process_button_string = tep_draw_hidden_field('x_Login', MODULE_PAYMENT_AUTHORIZENET_LOGIN) .
							   tep_draw_hidden_field("x_delim_char","|") .
								tep_draw_hidden_field("x_delim_data","TRUE") .
								tep_draw_hidden_field("x_url","FALSE") .
							//	tep_draw_hidden_field("x_test_request","TRUE") .
								tep_draw_hidden_field("x_type","AUTH_CAPTURE") .
								tep_draw_hidden_field("x_password",MODULE_PAYMENT_AUTHORIZENET_TXNKEY) .
								tep_draw_hidden_field("x_relay_response","FALSE") .
                               tep_draw_hidden_field('x_Method', ((MODULE_PAYMENT_AUTHORIZENET_METHOD == 'Credit Card') ? 'CC' : 'ECHECK')) .
                               tep_draw_hidden_field('x_Version', '3.1') .
                               tep_draw_hidden_field('x_Card_Num', $this->cc_card_number) .
                               tep_draw_hidden_field('x_Exp_Date', $this->cc_expiry_month . substr($this->cc_expiry_year, -2)) .
                               tep_draw_hidden_field('x_Amount', number_format($order->info['total'], 2,'.','')) .
                               tep_draw_hidden_field('x_Cust_ID', $FSESSION->customer_id) .
                               tep_draw_hidden_field('x_Email_Customer', ((MODULE_PAYMENT_AUTHORIZENET_EMAIL_CUSTOMER == 'True') ? 'TRUE': 'FALSE')) .
                               tep_draw_hidden_field('x_first_name', $order->billing['firstname']) .
                               tep_draw_hidden_field('x_last_name', $order->billing['lastname']) .
                               tep_draw_hidden_field('x_address', $order->billing['street_address']) .
                               tep_draw_hidden_field('x_city', $order->billing['city']) .
  tep_draw_hidden_field('x_state',tep_get_zone_name($order->billing['country_id'],$order->billing['zone_id'],STORE_ZONE)).
                               tep_draw_hidden_field('x_zip', $order->billing['postcode']) .
                               tep_draw_hidden_field('x_country', $order->billing['country']['title']) .
                               tep_draw_hidden_field('x_phone', $order->customer['telephone']) .
                               tep_draw_hidden_field('x_email', $order->customer['email_address']) .
// add hjmsoftware invoice number
								tep_draw_hidden_field('x_invoice_num', $new_order_id) .

                               tep_draw_hidden_field('x_ship_to_first_name', $order->delivery['firstname']) .
                               tep_draw_hidden_field('x_ship_to_last_name', $order->delivery['lastname']) .
                               tep_draw_hidden_field('x_ship_to_address', $order->delivery['street_address']) .
                               tep_draw_hidden_field('x_ship_to_city', $order->delivery['city']) .
tep_draw_hidden_field('x_ship_to_state',tep_get_zone_name($order->delivery['country_id'],$order->delivery['zone_id'],STORE_ZONE)).
                               tep_draw_hidden_field('x_ship_to_zip', $order->delivery['postcode']) .
                               tep_draw_hidden_field('x_ship_to_country', $order->delivery['country']['title']) .
                               tep_draw_hidden_field('x_Customer_IP', $FREQUEST->servervalue('REMOTE_ADDR')) .
                               $this->InsertFP(MODULE_PAYMENT_AUTHORIZENET_LOGIN, MODULE_PAYMENT_AUTHORIZENET_TXNKEY, number_format($order->info['total'], 2,'.',''), $sequence);
      $process_button_string .= tep_draw_hidden_field($FSESSION->name, $FSESSION->ID);
      if (MODULE_PAYMENT_AUTHORIZENET_TESTMODE == 'Test') $process_button_string .= tep_draw_hidden_field('x_Test_Request', 'TRUE');


      return $process_button_string;
    }

this also works for the Paypal modual.
Hope this helps
Jay