Hopefully we can get Martin (an Engineering Manager) to join this forum soon. Meanwhile, he passed along this method of adding another layer of security:
Our payment vendor FastSpring offers an extra layer of security to a paid plugin through license codes. When setup, purchasers will receive a unique license code in their receipt email to ensure your plugin is unlocked only by them.Use the following link to learn how to generate and upload a License Fulfillment (large list of unique license codes) for your new plugin.
Add a Fulfillment to your Product | FastSpring Classic Documentation
We recommend you submit your Plugin listing for Adobe as “Manually publish later” so that once approved by Adobe you can create the and upload the license codes through FastSpring before you publish your listing.For FastSpring questions about fulfillment codes: support@fastspring.com
Also note that you should create a large number of license codes because when they run out, you will have to contact Adobe to manually upload additional license codes. ccintrev@adobe.com
In that documentation there’s a POST method description, too.