Plugins and add-ons that came out of client work and were then packaged so any team can install them. Each one is licensed annually, updated by the people who wrote it, and documented on its own page before you spend anything.
Every product states what it does, what it depends on and what happens when a licence lapses, before any payment step.
Licences are annual and priced by the number of sites. Payment is taken by card through Stripe, on its own hosted checkout page.
Install the plugin on your site, enter your licence key and configure it. The product page documents the settings.
Support for a licensed product goes to service@trynextstep.com. If you are already an NSDS client, raise it in the client portal instead.
Most of what we build is commissioned rather than bought off a shelf. If the requirement is specific to your operation, that is a build conversation.