Create custom query for search?

Have you check your result-set, what result you are getting in $query obj. another thing, there is no need to use 'relation' => 'OR', in meta query array, have a try with this as well. And instead of 'numberposts' use 'posts_per_page' this will help. 🙂 or you can remove the meta_query and make your $arg something like

$args = array(
's' => $keyword,
'post_type'=> 'store',
'posts_per_page' => -1,
'meta_key' => 'postal_code',
'meta_value' => $keyword,
'meta_compare' => 'LIKE'
);