Should we worry about the PESA bus being full?

Ask your bnb owner if they can print your tickets before you buy them.
Go to the public library they usually have computers free to use and you pay to print or find an internet cafe (i was surprised to see these still exist in London)or ask the bnb owner where you can print something.

Jazz is right. We've been surprised at how we've always been able to get to a computer when we travel. It's not as easy as it was a few years ago. but if your landlords can't help you, try public libraries or the nearest TI. Or as you suggested, the bus depot may be able to help.

Try Rymans stores, they have printing services and they’re all over London. ryman.co.uk Also, google “internet cafe near me” or near your bnb post code. As others have said there are more Internet cafes around than we’d think. Even on a bank holiday you should be able to find somewhere.

You’re so smart to plan ahead for that bus to San Sebastián. Have a great last day in London!

Keri, in reviewing your post, I realize I never addressed your main question, should you book the bus in advance. Although I’ve not taken this bus, I’ve made a lot of connections from planes to buses over the years. The major variables are if the plane is delayed and/or if you have checked luggage to collect, plus time for reaching the bus stop. Perhaps you’ve already looked at the PESA fare conditions: PESA fare

Essentially you have to cancel or modify your tickets 2 hours before the departure time. Not sure how many are traveling but at €17 per ticket, is it worth the gamble to book the 16.45 rather than the 17.45. If you book the 17.45, you could always try to get on the earlier bus if there’s room. The downside of the 17.45 is arriving an hour later in San Sebastián, so it could be worth the gamble. If it’s just two of you, I’d be tempted to gamble.

Thanks to all for the replies -- very helpful (and more importantly, calming!) Barbara, you're right - the peace of mind is worth the smallish price. Our flight lands at 15:45 so unless something goes terribly amiss*, we should be safe booking the 16:45 bus. And we'd certainly not enjoy needing to go on the 17:45 bus if that can be avoided.

I've been checking for what's open today in London, given the Bank Holiday. It does look like the libraries may be closed. Ryman's was one I'd found initially but their website has one of those now-required cookie acceptance banners and, for some reason, it's not clickable in Safari so I had passed on them last night. I tried again using Chrome and that worked. It looks like they're open today, and they have the kind of printing services we need.

Our AirBnb host isn't local (and we've already bothered him enough about plumbing issues) so I think I'll pass on tapping him yet again. It looks like we have a few viable alternatives.

*On things that can go amiss: our rail to Southend Airport is now posted as having a 'disruption' (due to a week's repairs to the rail) between now and June 2. So our 44 minute journey tomorrow has turned into an hour-and-a-half, part bus/part rail.

FYI, we've now booked PESA online and, to answer part of my original question:

At the end of the transaction, I supplied my email and a confirmation was sent to me. It contained a QR code, with this message (translated):
"You can print your ticket by zooming this QR code in the light of the top of the readers installed next to the Bilbao, San Sebastián and Vitoria-Gasteiz ticket offices."

We also were provided PDFs of the tickets and will print a copy when we find a print facility. But it's nice to know the PESA booking system provided this alternative.

Keri, thanks for following up.