I hope someone has information on this for Egbert.
In 2007 I took the bus from Santiago to Porto after my Camino to do a few days of sightseeing. I didn't have a guidebook with me but knew the old town was fairly small and planned to get a map when there. Unfortunately, the bus didn't go to the Porto bus station as I had assumed it would but dropped us all off on a street corner somewhere. It was Sunday, everything was closed, no one spoke english (and I knew NO portugese), all those who also got off the bus were busily being picked up by their organized rides - I spun around in circles and almost cried. Where was I? Luckily a taxi driver approached (at least I think he was a taxi tho nothing on his car nor in his car advertised such) and in pantomime promised to take me to a cheap hotel. Which he did, the hotel was fine and it all worked out.
Hi, the bus station is marked on the map of Porto on the Let's Go-- Spain and Portugal book. I was using the 2006 book. It's about a 15 minute walk at most from downtown Porto. I got off, grabbed my rucksack and walking stick and simply walked over to the place where I had reservations. Porto is a nice town. I took the ride in reversed. I had walked up to Santiago along the Portuguese route and took the bus back to explore the city and catch the Ryanair flight to Paris.
I took the ALSA bus last summer and the bus stop was at Praca De Galiza 96 and when I checked the ALSA website just now (use the Internernational Services tab) they are showing the same stop. Since the bus arrived after dark and my hotel was near the train station I took a cab.