Project Export for Jira Cloud is provided under Atlassian's standard Marketplace end user agreement (atlassian.com/licensing/marketplace/end-user-agreement-v1), together with the provider-specific terms on this page. If they conflict, the end user agreement wins unless it lets these terms say otherwise.
You buy and pay through the Atlassian Marketplace; Atlassian bills you, and Atlassian's Marketplace refund rules apply.
The free trial exports a project's full setup and its first 100 work items, so you can check the file before you pay. A paid licence exports every work item.
What the app does, and doesn't do
The app makes a file of one Jira Cloud project. It is not a scheduled backup of your site, and it doesn't replace Atlassian's own backups.
Export files are deleted 7 days after an export finishes. Download what you want to keep.
The restore script is a separate, optional tool that you run yourself with your own API token. What a restore keeps and what Jira doesn't let anyone write back is listed in each export's README and on the docs page. Test a restore in a project you can delete before you rely on it.
Support
Support is by email, as described on the support page.