Plugin licenses are perpetual - you only have to purchase them once.
We have never sold a trial extension, but we have issued them to users who have trialled plugins previously. For example, when we released version 2.0.0 of Archetype: Abasi, we gave every user who has previously trialled it a 14-day trial extension. We have done this several times.
If you received a trial extension for a plugin you already purchased, make sure the purchased license is the one that is activated in iLok License Manager. If it is, and you’re having issues with launching the plugin, please reach out to our support team at support@neuraldsp.com.
There should be no issues with iLok and trial extensions though. Hope that helps!