// Delect Button Click
 private void customerDeleteButton_Click(object sender, EventArgs e)
 {
     // If selected ID is valid
     if (selectedCustomerID != -1)
     {
         // Delete customer
         DataInterface.deleteCustomer(selectedCustomerID);
     }
     else
     {
         // If not valid, display message
         MessageBox.Show("Please select a customer to delete and try again.");
     }
     // Re-display data
     customersDGV.Refresh();
     displayCustomers();
 }