eCart does not generate an invoice number, but it does generate an order number which is the ID for the record created in the orders table.
When you run the checkout wizard, make sure to select the option to save the order info to the database.
When the info is saved, a new record is stored in the order table, and the Order ID is set to use the ID value for the primary key column in the orders table.
If you use a local checkout method like paypal pro, eCart can also send am email receipt that will contain the order ID value.
you can create a recordset lookup to search for the order IDF and the Zip code.