I just had a look for flight tickets (one in May, one in June) -
flight from Amsterdam to Biarritz . No direct flight & prices (one way) are from 550€ upwards.
flight from Amsterdam to Bordeaux . Air France direct (but expensive). British Airways via London at a reasonable price (opodo.be)
From Bordeaux, you still have to take a train to Bayonne. And from Bayonne to Saint Jean.