While I have heard different stories over whether or not a do-it-yourself credencial is received in Santiago, remember that the point of the document is to identify you as a bona fide pilgrim rather than a tourist. Membership in a national pilgrims organization is considered to be sufficient indication, as is requesting it from an official source (cathedral, turismo, pilgrims' office, albergue). Best to obtain a kosher document, and take away any doubt or ambiguity.