Remind that if you book the ALSA bus on line on beforehand, they do NOT accept foreign creditcards. only spanish cards are accepted as we unfortunately found out.
You can pay by Pay pal so if you do ,do not forget your pay pal password. Crazy system of ALSA. If you call them, nobody speaks english !
At last we booked the bus through the Santiago Tourist office, allmost next door of the Santiago Camino office where you obtain your Compostela certificate
the Tourist office charges you a little fee for booking the bus. you also can book at the bussation which is about 20 minutes walk from the center of Santiago but better is booking in advance as soon as you 'll reach Santiago . .It can be busy at the busterminal and maybe the bus is fully booked.
Taking the train is also possible but takes a long time traveling by Vigo and there you have to change because Spanish trains have wider railtracks than Portugese trains. so the best is to take the bus. Takes you about 4 hours to get from Santiago to Porto.