Licenses are installed after the app is installed using the Atlassian Universal Plugin Manager (UPM).
If your evaluation license is going to expire before the purchasing process is complete go to the Marketplace entry for the add-on and press Free 30 Day Trial to get another evaluation license. You can then install the new evaluation license on your instance via Manage Add-ons in Jira.
Community/Open Source Software (OSS) licenses for apps are available free by emailing email@example.com and requesting a Community/OSS license.
If you have any other licensing problems please contact Atlassian via email at firstname.lastname@example.org.