well, to do this with the client_id, you would need to force the customer to log in before they can checkout using security assist.
you could set up the users table so that the primary key column starts at 135, when the user logs in, the ID is stored in a session, you could use that session in the x_cust_id form element.
see the "Applying Register/Login to your checkout" tutorial on the eCart support page for details:
you have the order info being stored in the database on the confirm page. Set the OrderID column of the orders table to start at 11001 and use the BillCart_OrderID session in the
<input type="hidden" name="x_invoice_num" id="x_invoice_num" value="<?php echo(isset($_SESSION['BillCart_OrderID'])?$_SESSION['BillCart_OrderID']:""); ?>" />