Problem inserting Data to Database in ASP.Net c#

Hi,

i try to insert every item from a class into a database.
When i run the program, the Data shows up in SQL, but the application throws the following exception:

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

What can i do to fix it?

protected void btnCheckout_Click(object sender, EventArgs e)
        {
            ADDTest.Datasets.dsCribstoreTableAdapters.CheckoutTableAdapter dsCheckout = new ADDTest.Datasets.dsCribstoreTableAdapters.CheckoutTableAdapter();
            int intItems = ShoppingCart.Instance.Items.Count;
            for (int i = 0; i <= intItems; i++)
            {
                dsCheckout.Insert((string)ShoppingCart.Instance.Items[i].USer,(string)ShoppingCart.Instance.Items[i].ProductId,(int)ShoppingCart.Instance.Items[i].Quantity,(string)ShoppingCart.Instance.Items[i].ChrgDepartment,null,DateTime.Now);
            }

        }

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