If you can cancel the booking, its much cheaper to get the bus to the Biarritz railway station (not the Bayonne station), take train to Hendaye on the border then the local Euskatren to Donostia/San Sebastion. Walk to bus station at top of road where the Euskatren drops you and get the bus to Pamplona. Sounds complicated but its fairly simple and a whole lot cheaper than taxi. You may find it difficult to get 5 others at that time of year. I made the same journey late March 2013 and there was only one other person going to Pamplona and three going to St Jean. By the way if you arrive too late to get into albergue, Hostel Hemingway is excellent