While the advice is good as another option, IMHO it presents an extra headache in the "extra step.
Get to Biarritz (presumably by plane)
Take a bus from Biarritz to Bayonne
Take a train (or substitute bus) from there to SJPdP
We've all been...
If you arrive by train in Bayonne too late to get a local train to SJPdP you might stay in the pilgrim albergue in Bayonne for the night. They take reservations. See more info in this Gronze citation. You could then catch the first local train in...
There is a regular local bus from Biarritz airport to Bayonne railway station. The Express Bourricot shuttle can be expensive (€ 130) if there are no other passengers! Train fom Bayonne to SJPdP is cheap (approx. € 12).
There are a number of Transavia France flights from Paris-Orly to Biarritz (BIQ-Pays Basque) for $70 to 90 USD. Flight time is about 1 hours and 20 minutes. There are bus, train, and shuttle services that can get you from there to SJPdP (I...
www.rome2rio.com
In short :
1) metro to Gare Montparnasse (ticket from vending machine in metro station)
2) from there TGV (fast train) to Bayonne. Book ticket in advance...
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