No default selection in a select item on commerce checkout pages using hook_form_alter Thanks to the help I received in my previous question, I was able to change the HTML of shipping service from radios to a select field. This was done, as suggested, by means of a custom module implementing hook_form_alter.    

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id){
    if ($form_id=='commerce_checkout_form_checkout') {
        $form['commerce_shipping']['shipping_service']['#type']='select';
    }
}    


Now what i need is to have an empty option ("select the proper value") in it, so that the user would be forced to actively choose one of the shipping methods. 
What I get from     

print_r($form);


is     

    [shipping_service] =&gt; Array    
                (    
                [#type] =&gt; radios    
                [#options] =&gt; Array    
                    (    
                        [deli] =&gt; Delivery : 5,00    
                        [pickup] =&gt; Pickup: 0,00
                    )
                [#ajax] =&gt; Array
                    (
                        [callback] =&gt; commerce_shipping_pane_service_details_refresh
                        [wrapper] =&gt; commerce-shipping-service-details
                    )

                [deli] =&gt; Array
                    (
                        [#description] =&gt; 
                    )

                [pickup] =&gt; Array
                    (
                        [#description] =&gt; 
                    )

                [#default_value] =&gt; deli
            )

        [service_details] =&gt; Array
            (
                [#type] =&gt; container
                [#prefix] =&gt; 

                [#suffix] =&gt; 

            )         


I tried to tinker with dafault value, inserting another option, like      

$form['commerce_shipping']['shipping_service']['#options']['_none']='['-select-']';
$form['commerce_shipping']['shipping_service']['#options']['#default_value']='['_none']';


(which was somewhat randomly inspired by the 'print_r' output from a taxonomy term field on the same form  but brought no results and did cause AJAX errors. 
Any ideas?
The documentation says:


  If #required is TRUE and there is no #default_value, an empty option is added to the select control to force the user to make an active choice.


So take the default value out and make sure that the form code includes '#required' =&gt; TRUE,

I also read somewhere that you must use drupal_get_form() to build the output for a form otherwise the additional form processing functions will not run.  I did not use hook_form_alter() to modify shipping services but instead used hook_commerce_shipping_service_info() in a custom module to give me more flexibility.