There is a one time sign up fee and a low monthly fee, both are per user.
There are four different monthly rates based on the number of users subscribed at a single time, for a single store. Users bought together cannot be split among different stores. For example, if you have two stores and need one user per store buy two single user packs.
If you are adding users to an existing store contact us to cancel the existing subscription and start a new one if it will move you to a lower price per user.