Hi Nadya,
Each user must have an account before given access to the shopping cart. Once the user fills out the account form, have the system auto-increment an account number. Then the user can enter his account number and some other data like email address, mother's maiden name, etc to access the system. All shopping cart transactions should have the user account number as a foreign key that relates back to the parent table of customer's accounts.
You might have the user enter his account number when checking out. This would be necessary before the order could be processed.
LelandJ