Each website or subdomain requires its own license to use FooSales. So for example, if your setup is https://subdomain1.yoursite.com, https://subdomain2.yoursite.com etc. OR https://www.yoursite.com/site1/, https://www.yoursite.com/site2/ etc. you will need to purchase a Multiple Domain Plan.
If you have a single website/domain and would like to test FooSales on a staging site from time to time, you can do this with a Single Domain Plan by linking your FooSales Account to the staging site (e.g. https://staging.yourstore.com) and then link it back to your production site when you have finished testing (https://www.yourstore.com)
Refer to the Pricing page for more info: https://www.foosales.com/pricing/