Credencial
I haven't even started my first Camino yet - so probably not the best source of advice, but I applied to Mundicamino by post a week ago (the form and instructions - in Spanish - are on
http://www.mundicamino.com) and received my credencial today. I am doing a crash course in Spanish, so filled the form out with dictionary in hand! The credencial is 5 Euros for EU citizens, 10 for the rest of the world. I sent cash, secured according to their instructions.
Otherwise look at the replies to my previous query (obtaining one in Burgos), and on the Mundicamino site.
Good luck!