OK. To save about a fiver I got my glasses delivered to a "relay point". Yes. I know. There are various networks of relay points in France and the online opticians use a particular one that has addresses in Mérignac, in various places dotted about Bordeaux and at the main railway station, Gare Saint-Jean. None in Pessac. I chose Gare Saint-Jean.
Now to get from Pessac to Gare Saint-Jean can take anything between 5 minutes and an hour. Trains from Pessac go there directly without passing go, the journey is not included in our monthly bus and tram card but it costs about 2€. At 15:00 I got the message that the specs were ready for collection. There wasn't a train for a couple hours.
So at about 4 I took the bus. Bus 4 to Barrière de Pessac, then bus 11 to Gare Saint-Jean. It was pouring with rain, so the buses were crowded and steamy, but we got there OK. And it took about an hour including perhaps 15 minutes hanging around at bus stops. Collecting the glasses took a little while. To come back there was a train at 18:00, so I bought a ticket and a burger from MacDonalds and assumed my new identity as a mild-mannered, nordic theologian with his signature round plastic specs.