EDD – Free download (Simple)
$meta_query = array( 'relation' => 'OR', array( 'key' => 'edd_price', 'compare' => 'NOT EXISTS', ), array( 'key' => 'edd_price', 'value' => 'zero', ), );
$meta_query = array( 'relation' => 'OR', array( 'key' => 'edd_price', 'compare' => 'NOT EXISTS', ), array( 'key' => 'edd_price', 'value' => 'zero', ), );