Our system is a completely free hosted website platform with ecommerce. But you are welcome to keep your existing website and there are 3 ways to do this.
Embed our store into your website using an iframe (note we do not integrate with Shopify yet but we are working on this)
Use a subdomain for the shop. ie. shop.yourdomain.com.au
Or use a reverse proxy. See the post about adding a blog to your website for information on how to do a reverse proxy.