In Atlassian Cloud terms, you buy an add-on by subscribing to it. An active add-on subscription activates the features of the add-on in your Jira Cloud instance. To try or subscribe to an add-on, click the Free Trial button next to the add-on in the Manage Add-ons page.
You click this button whether you're ready to subscribe or just want to try the add-on for now. If you do not terminate the trial within the trial period, a subscription for the add-on kicks in. Trials last for a month from the start of your next billing date. After starting a trial, you can stop it from the add-on administration page.
Although you can re-enable the trial within the original trial period, you cannot try it again beyond the original trial period. At that point, you will only be able to subscribe to the add-on from the Manage Add-ons page.