This is really easy to acheive.
Simply have a "customers" table in your database. When created each customer will have a unique ID.
Using Security Assist create a login page, and create a rule to password protect your checkout page so that the customer has to login to access it. With security assist authentication create a session variable to store the unique customer ID.
THen in your orders table store the customer ID as a foreign key ID - thats the link between the customer and the order. You can then create relational queries or simply queries based on the value of the customer ID to recall the order by customer.
Hope this helps.