The national postal service is called Correos in Spain.
Correos en Santiago de Compostela (9). Sucursal de Correos en Santiago de Compostela. Operador español de servicio postal. Recogida y envío de paquetes y cartas. Envío de documentos, burofax, tarjetas postales y notificaciones administrativas. Cartas certificadas, urgentes y ordinarias. En...
www.paxinasgalegas.es
More than one office in Santiago de Compostela.
The most convenient ones will be the one in the Pilgrim's Office in Rua Carretas.
Other central one is at Travesia do Franco.
You are referring to a mail? And then asking for a postal office. Do I understand that you have to print out the mail and send it back through regular mail?