Hello, Lysandre! (and Hi, Eric!)
Yes, you should be able to buy a sandwich at Refuge Orisson. I did that in 2010 when I stayed overnight there. If I remember correctly, it was tomato and cheese (and ham was also available).
Last year I walked from SJPP to Roncesvalles in one day, but I stopped at Orisson to eat an early lunch... which was bread with tomato and cheese I'd bought in SJPP the night before
. There were several pilgrims who went inside and ordered a sandwich then, to eat later that day, and there were others like me who had bought food in SJPP and ate that.
As Margaret wrote, there are many places along the main street in SJPP where you can buy a meal. It was fun for me to walk past and look in the windows, read the menus and imagine what I'd choose! I went to a small grocery shop almost at the end of the street, however, and bought food to cook at the albergue (and for lunch).
Of course you will be ok when you get there! Feeling nervous is understandable, but it will all make sense and you can ask for help if you need to. You really can't get it wrong, and if you're anything like me, chocolate always helps
.
Buen Camino!
Rachel (Caminoheart)