It's optional. If you do not purchase your own certificate, the secure areas of the store (when gathering payment information) will be hosted by a shared secure server (e.g. store.serverbox.com). The URL in the browser will look something like https://store.serverbox.com/your-client/etc/etc. When your customers are outside the secure area of the store, it will show their own domain name in the browser's URL. Regardless of whether or not you choose to get your own SSL certificate, the web based interface for setting up and maintaining your on-line store remains the same.