If you decide to continue, you'll get billed once every month. The price is 5$ pr. month, pr. agent.
You only pay for the agents who have access to this app. So if you've restricted the use of the app to a certain group of agents, you'll only pay for that group of agents.
For instance, if you have 10 agents in Zendesk, but only 4 of them should've access to the app. Create a group in Zendesk and restrict the use of the app to that group.