i volunteered at my local charity shop for a year, only once a week. but it was worth it.
sad to say i can imagine it depends what kind of charity shop, I worked at a local hospice shop, but I imagine if you work for Oxfam it would look better, purely because its an international charity. That said it is a great opportunity to learn skills, as long as you make sure you put on your CV that you worked for nothing I think it looks great. and a great experience and way to ease boredom