Inserting records into a database
This is not best practice but simple logic.

Why you have add inside the loop this (non change) parameters ?

    string Type = "D";
    DateTime Date = DateTime.Now;
 string connectionString = 
       ConfigurationManager.ConnectionStrings["Foo"].ConnectionString;


Get them out of the loop.

Second place the connection inside using, and totally outside the loop, why you need to open it and close it all the time.

protected void RadButton2_Click(object sender, EventArgs e)
{
    //GridDataItem item = (GridDataItem)RadGrid1.SelectedItems; 
    string Type = "D";
    DateTime Date = DateTime.Now;

    string connectionString = ConfigurationManager.ConnectionStrings["Foo"].ConnectionString;
    SqlConnection connection = new SqlConnection(connectionString);

    using(SqlConnection connection = new SqlConnection(connectionString))
    {
        connection.Open();
        foreach (GridDataItem item in RadGrid1.SelectedItems)
        {
            try
            {
                string Description = "Monthly Storage Fee - Tag: " + item["PackageTag"].Text + Label3.Text;
                Int32 AmountDue = Convert.ToInt32(item["AmtDue"].Text);
                int UserID = Convert.ToInt16(item["UserID"].Text);

                using (SqlCommand cmd = new SqlCommand("INSERT INTO Billing (UserID, Type, Date, Description, Amount) VALUES (@UserID, @Type, @Date, @Description, @AmountDue)", connection))
                {
                    cmd.Parameters.AddWithValue("@UserID", UserID);
                    cmd.Parameters.AddWithValue("@Type", Type);
                    cmd.Parameters.AddWithValue("@Date", Date);
                    cmd.Parameters.AddWithValue("@Description", Description);
                    cmd.Parameters.AddWithValue("@AmountDue", AmountDue);

                    cmd.ExecuteNonQuery();
                }
            }
            catch
            {
                Label4.Text = "uh oh";
            }
        }    
    }
}


Need some extra test for check if the connection is open or fail