в нужном шаге добавляете условие isset($_POST['operation_id']) пришол ли запрос с яндекса
дальше разбираете. реализовано тут
http://www.viofit.ru/shop/cart/
if (isset($_POST['address'])){
$ya_address=json_decode(urldecode($_POST['address']));
$external_propertys['site_users_name'] = $ya_address->{'firstname'};
$external_propertys['site_users_surname'] = $ya_address->{'lastname'};
$external_propertys['site_users_patronymic'] =$ya_address->{'fathersname'};
$external_propertys['site_users_email'] = $ya_address->{'email'};
$external_propertys['site_users_phone'] = $ya_address->{'phone'};
$ya_compl_address=$ya_address->{'country'}." ".$ya_address->{'city'}." метро ".$ya_address->{'metro'}." ";
$ya_compl_address.=$ya_address->{'street'}." д ".$ya_address->{'building'}." корп ".$ya_address->{'suite'}." подъезд ".$ya_address->{'entrance'}." этаж ".$ya_address->{'floor'}." домофон ".$ya_address->{'intercom'}." квартира ".$ya_address->{'flat'};
$ya_compl_address.=" комментарий к адресу ".$ya_address->{'comment'};
$external_propertys['site_users_address'] = $ya_compl_address;
$external_propertys['site_users_postcode'] = $ya_address->{'zip'};
$ya_metro=$ya_address->{'metro'};
}