I have used the travel agency of "El Corte Ingles" in Santiago several times for booking flights and have always gotten good service.
I am not 100% sure if they book buses, but you could contact them and ask. I know they speak English, and they are professional and trustworth.
Viajes El Corte Ingles
C/ Rosalia de Castro nÂº 1 Bajo
ROSALIA DE CASTRO
Santiago de Composte
Tel. 981558020 . Fax 981575702
They have an office in the Calle Rosalia de Castro, which is the continuation of Rua da Senra. You will find this street in the bottom left corner of this map:
... the agency has an office 200 meters from where the map stops (direction up and left on the map).
They also have a website, but it seems to be down right now. Try to access it here for bus reservations:
http://www.elcorteingles.es/viajes/VECI ... utocar.htm
and see if you can do any of this online.
Let me know how it goes, :wink: