You're going to have a happier mailing list if you allow people to opt in rather than have to opt out of something they didn't choose.
You might choose to send out an initial mass mailing to your customers, announcing something new about your shop (a contest, a new line of items) and use that as an opportunity to say you'll be emailing new info x number of times a year (I never do more than 6), and if they want to be on that list to be the first to know about new things, you might get a nice % of your people opting in.
I have several bead artisans who send me monthly newsletters with their new products, but I'm good with that since they first gave me the chance to buy in.
Happy customers!