My suggestion would be to take three days from St. Palais to walk to Honto on the flank of the Pyrenees, i.e. do a 400 metres ascent on your third day (almost one third of what you have to do to get to the top and the Spanish border). Spend the night in Honto, and two if you are already tired. From there, the Pyrenees are not really that hard if you take it easy, i.e. if you walk at your own speed and don't try to keep up with anyone else. This is assuming that you don't carry one of those really heavy rucksacks.
On my walk from Honto, I encountered quite a few people who had started in StJPdP but didn't get further than Roncesvalles. One guy was walking with a 16kg rucksack, others sped up really fast, and a young couple were trying to do the walk in medieval style sandals. An overweight American lady had to be taken back down the mountain due to exhaustion.
So, whether you start in St. Palais or StJPdP, my suggestion is that you spend a night in Honto. I am sure you will be okay.
I don't know whether you'll get a credential and/or a shell in St. Palais. If you can get them at home (or online) you'll avoid the hassle of looking for them in France. As a weight minimisation freak, I got my shell from Felisa near Logrono (her daughter now sells the shells and other stuff by the side of the Camino on the descent just before Logrono). A credential however you should have in St. Palais already.
I would book ahead for your one night in St. Palais, as it is on a minor pilgrim route. St. Palais is about 3km from the main route, the Via Podiensis. On that route, you will meet other pilgrims, especially if you stop when you get to the Via Podiensis, at the Beneditenia stone (3 French caminos merge into one there). Most other pilgrims will have a Miam-miam dodo or a similar publication with information about the route to the Spanish border. If you have a smart phone, ask them whether you can take pictures of accommodation information. Accommodation tends to fill up in the French Basque country, especially during the main season, so as soon as you have the information, make a reservation for the following night.
If you want to take along a detailed map from Beneditenia to StJPdP, go to this site:
http://www.jakobsweg.ch/index.php/en/eu/frankreich-en-US/jakobswege-en-US/f-via-lemovicensis-b/
Click to enlarge the map and play around with the "Karten" function at the top right. Enlarge to what suits you and take as many "screen capture" pics (e.g. with PhotoScape) as necessary (I usually print on both sides of the paper to save a few more grams).
The map for the route from StJPdP to Roncesvalles you'll find here:
http://www.jakobsweg.ch/index.php/e...a-roncavalles-leon/st-jean-p-d-p-roncevalles/
Buen Camino!!!
Walter