You need to BACKUP!! BACKUP!! BACKUP!! BACKUP!! then find a bit of code in catalog/includes/modules/payment/authorizenet_cc_aim.php that looks like this:

function before_process() {
global $HTTP_POST_VARS, $customer_id, $order, $sendto, $currency;

$params = array('x_login' => substr(MODULE_PAYMENT_AUTHORIZENET_CC_AIM_LOGIN_ID, 0, 20),
'x_tran_key' => substr(MODULE_PAYMENT_AUTHORIZENET_CC_AIM_TRANSACTION_KEY, 0, 16),
'x_version' => '3.1',
'x_delim_data' => 'TRUE',
'x_delim_char' => ',',
'x_encap_char' => '"',
'x_relay_response' => 'FALSE',
'x_first_name' => substr($order->billing['firstname'], 0, 50),
'x_last_name' => substr($order->billing['lastname'], 0, 50),
'x_company' => substr($order->billing['company'], 0, 50),
'x_address' => substr($order->billing['street_address'], 0, 60),
'x_city' => substr($order->billing['city'], 0, 40),
'x_state' => substr($order->billing['state'], 0, 40),
'x_zip' => substr($order->billing['postcode'], 0, 20),
'x_country' => substr($order->billing['country']['title'], 0, 60),
'x_phone' => substr($order->customer['telephone'], 0, 25),
'x_cust_id' => substr($customer_id, 0, 20),
'x_customer_ip' => tep_get_ip_address(),
'x_email' => substr($order->customer['email_address'], 0, 255),
'x_description' => substr(STORE_NAME, 0, 255),
'x_amount' => substr($this->format_raw($order->info['total']), 0, 15),
'x_currency_code' => substr($currency, 0, 3),
'x_method' => 'CC',
'x_type' =>

And Replace it with this:

function before_process() {
global $HTTP_POST_VARS, $customer_id, $invoice_id, $order, $sendto, $currency;

$next_inv = '';
$inv_id = tep_db_query("select orders_id from " . TABLE_ORDERS . " order by orders_id DESC limit 1");
$last_inv = tep_db_fetch_array($inv_id);
$next_inv = $last_inv['orders_id']+1;

$params = array('x_login' => substr(MODULE_PAYMENT_AUTHORIZENET_CC_AIM_LOGIN_ID, 0, 20),
'x_tran_key' => substr(MODULE_PAYMENT_AUTHORIZENET_CC_AIM_TRANSACTION_KEY, 0, 16),
'x_version' => '3.1',
'x_delim_data' => 'TRUE',
'x_delim_char' => ',',
'x_encap_char' => '"',
'x_relay_response' => 'FALSE',
'x_invoice_num' => substr($next_inv, 0, 20),
'x_first_name' => substr($order->billing['firstname'], 0, 50),
'x_last_name' => substr($order->billing['lastname'], 0, 50),
'x_company' => substr($order->billing['company'], 0, 50),
'x_address' => substr($order->billing['street_address'], 0, 60),
'x_city' => substr($order->billing['city'], 0, 40),
'x_state' => substr($order->billing['state'], 0, 40),
'x_zip' => substr($order->billing['postcode'], 0, 20),
'x_country' => substr($order->billing['country']['title'], 0, 60),
'x_phone' => substr($order->customer['telephone'], 0, 25),
'x_cust_id' => substr($customer_id, 0, 20),
'x_customer_ip' => tep_get_ip_address(),
'x_email' => substr($order->customer['email_address'], 0, 255),
'x_description' => substr(STORE_NAME, 0, 255),
'x_amount' => substr($this->format_raw($order->info['total']), 0, 15),
'x_currency_code' => substr($currency, 0, 3),
'x_method' => 'CC',
'x_type' =>