I want to buy a bus ticket on www.pesa.net but there's no option on the drop down menu for San Sebastian. Am I missing something here?
Audra, I haven't done this trip. Since you don't have any responses, I'll give you my best guess. Spanish bus stations aren't like US ones, they're more like the Tijuana station - mulitple competing bus lines that operate from the same station - like an airport. Inside you will find several ticket windows for the different lines and an information booth. Busses are cheap and rarely sell out. You don't need to book ahead to get either a good deal or a seat. I like to buy my ticket either when I arrive in town or the afternoon before I leave (if I'm not sure when I arrive). I don't know if you can get a bus directly to the Bilbao airport from San Sebastian. You may have to bus to Bilbao, then take another to the airport. The information desk can help you figure it all out - but English won't be their strong suit (and Mexican Spanish isn't understood particularly well either).
The bus goes from Plaza Pius XII. It's not a station, just the circular plaza where buses from many outfits drop off and pick up people. Nor are the tickets sold all in one spot. I don't know where the PESA office is. Try using Donostia-San Sebastián as the start point. Like Brad, I've never seen a full bus in Spain except maybe for the ones running around the cities at peak times.
We did the trip in reverse. The following link provides the details. no need to book in advance. http://www.spanish-airports.com/bilbao/buses.php
Thank you for the replies. I think I'll just play it by ear and buy the tickets once I'm there. I don't have to micromanage everything! :)