You can enter a custom subdomain for your E-junkie Shop URL -- e.g.,
yourname.e-junkie.com -- in your E-junkie Dashboard under Manage Cart & Shop > Shop Preferences.
Note that the custom Shop URL you enter there can only use letters, numbers and dashes (no spaces or other symbols/punctuation), and it can take a few minutes for your live Shop pages to pick up changes to your Dashboard settings.
Without entering a custom Shop URL, your default Shop URL just uses your E-junkie Client ID numbers, which will always be the same for you. That version of your Shop URL will always redirect to your current custom Shop URL, even if you change that later.