Lost In Procedures

This procedure is missing one importent piece is I need to have a customerID added to the invoice table.  The CustomerID comes from another table called Customer_Invoice_cart.  I don't know how to convert Customer_Invoice_Cart.CustomerID to a @CustomerID to add to the table. If there is anyone out there who is good at stored procedures (unlike me who is learning) could help me out.  FYI I realy cannot get the CustomerID for the .aspx page, it is not called any where.



procedure [dbo].[InvoiceAdd]
(
    @EmployeeID int,
    @CartID     nvarchar(50),
    @OrderDate  datetime,       
    @OrderID    int OUTPUT
)
AS

BEGIN TRAN AddInvoice

/* Create the Order header */


INSERT INTO invoice
(
    employee_number,
    invoice_date
)
VALUES
(  
    @EmployeeID,   
    @OrderDate
)

SELECT

    @OrderID = @@Identity     
   
/* Copy items from given shopping cart to OrdersDetail table for given OrderID*/

INSERT INTO invoice_books
(
    invoiceID,
    ProductID,
    TaxID,
    Quanatity,
    UnitCost,
    UnitPrice  
)

SELECT
    @OrderID,
    Inventory_Invoice_Cart.InventoryID,
    Inventory_Invoice_Cart.TaxID,
    Inventory_Invoice_Cart.Quanity,
    products.UnitCost,
    products.UnitPrice
   
FROM
    Inventory_Invoice_Cart
  INNER JOIN products ON Inventory_Invoice_Cart.InventoryID = products.ProductID
   

WHERE
    Invoice_CartID = @CartID

INSERT INTO invoice_books
(
    invoiceID,
    ServiceID,
    Service_TaxID,
    Service_Quanatity,
    ServiceCost
)

SELECT
    @OrderID,
    Service_Invoice_Cart.ServiceID,
    Service_Invoice_Cart.TaxID,
    Service_Invoice_Cart.Quanity,
    services.price_leval1
FROM
    Service_Invoice_Cart
  INNER JOIN services ON Service_Invoice_Cart.ServiceID = services.serviceID
   

WHERE
    Invoice_CartID = @CartID


/* Removal of  items from user's shopping cart will happen on the business layer*/
EXEC Customer_Invoice_Cart_Empty @CartID
EXEC Inventory_Invoice_Cart_Empty @CartID
EXEC Service_Invoice_Cart_Empty @CartID

COMMIT TRAN AddInvoice