Once your evaluation period is about to end, you will have to buy FindDuplicates license for your JIRA server.
You can buy FD license:
From JIRA Administration > Add-ons
Directly from Atlassian Marketplace > FindDuplcates page
Buy FindDuplicates License from JIRA Administration
Go to JIRA Administration > Add-ons
In Atlassian Marketplace section switch to Manage add-ons
Press Buy Now button → The system will redirect you to your Atlassian page (MyAtlassian)
Follow the instructions from MyAtlassian page to buy and activate FindDuplicates license for your JIRA Server.
In case you want to buy FindDuplicates plug-in license without evaluating the product, you can do it directly from Atlassian Marketplace.
Buy FindDuplicates License from MyAtlassian
Go to Atlassian Marketplace and search for 'Find Duplicates'
If you already installed FD to evaluate the product before, you can just go to MyAtlassian page
Open Find Duplicates page from Search Results
Click on Buy it now link
Follow the instructions on Atlassian page to order/buy FindDuplicates license.
The rules for FD plug-in pricing and maintenance are Atlassian-standard:
Server products and add-ons are hosted on your servers. Licenses are perpetual and the purchase price include 12 months of maintenance (support and version updates).
You can renew maintenance after 12 months at 50% of the current purchase price. You can upgrade the tier of your host product and add-on licenses at any time. Upgrade prices are calculated based on Atlassian's formula.
If add-on pricing changes after your initial purchase, there's a 60-day grandfathering period during which you can renew based on the old pricing.