I turned off the option with the popup in the menu page because I didn’t want it. If I activate this option then the option with self-pickup is indeed displayed under the drop menu.
What i have at the moment is the following:
I built an option on the start page using the shortcode [wppizza_dbp]. the customer has to decide on the start page which postal code he has to select.
The pickup option is displayed under the dropmenu. If the customer choose the option to selfpickup and then he clicked on “ok”, the page don´t switch to menupage.
Better it was when the option of selfpickup is part of the dropmenu.

Thanks for your answer Olly,
Sorry, i have forget to define the variable in my exemple above. Here is my next try:

add_filter('wppizza_filter_gateways_payment_options', 'my_custom_gateways_on_zero', 10 , 3);
function my_custom_gateways_on_zero($gateway_objects, $order,  $user_data){
$pluginSlug = 'wppizza_preorder';
$preorder =  wppizza_get_order_meta($order['ordervars']['order_id']['value'], $pluginSlug, true );
if($preorder !='asap'){
foreach($gateway_objects as $id => $obj){	
if($id !== 'COD'){
unset($gateway_objects->PAYPAL);	
}		
}
}
}

=>
Warning: get_object_vars() expects parameter 1 to be object, null given in /www/htdocs/xxx/xxx/wp-content/plugins/wppizza/classes/class.wppizza.gateways.php on line 428

Warning: count(): Parameter must be an array or an object that implements Countable in /www/htdocs/xxx/xxx/wp-content/plugins/wppizza/classes/class.wppizza.gateways.php on line 428

Warning: Invalid argument supplied for foreach() in /www/htdocs/xxx/xxx/wp-content/plugins/wppizza/classes/class.wppizza.gateways.php on line 437

Note, that when the user choose a time slot for the preoder in the order page (so the value is not more ‘asap’), the order page will not be updated!