I need to book a seat on a train from Barcelona to San Sebastian. The site I was using wants to send the ticket by post, but that will be too slow.
Is there a way I can do it electronically, or pick up the reservation at the station?
If you have a pass don't bother with reserving in advance. Do it at the station once you are there.
You should not have bought a pass however. Somehow the idea that a Railpass is needed for train travel in Europe seems to be very hard to eradicate… So people buy a pass first, and then investigate reservation. What you should do however, is investigate first what tickets costs.
The RENFE website will allow you to buy online tickets for some trains (Barcelona - San Sebastian) and those you can print out yourself. These tickets are, when booked in advance, usually a lot cheaper than a day on a railpass.