You would need to create a shipping address table. the shipping address table should contain a foreign key relationship to the users table:
shippingID - Pirmary Key
shippingUserID - Foreign key to users.userID
ShippingFirstName
shippingLastName
etc...
You will need to make sure that the user is logged in before they can add shipping addresses. see the "How to apply Login/Register functionality to your Checkout Process" tutorial in the How To section of the eCart support page:
ecart/
When the user logs in, a session variable will be created with the unique primary key from the users table. In the Add Shipping Address form, store that session variable in the shippingUserID column.
To display a select list with the different shipping addresses the have entered, create a recordset the queries the shipping address table. Filter the shippingUserID column on the userID session variable.