I was always using the Zendesk purchase code validator from Mattes (https://github.com/mattes/zendesk-envato-app) but it seems like this plugins isn’t getting updated anymore, and since the new support changes you can’t verify if the buyer is still in his support period.
So I made some changes the the script of Mattes and would like to share them with everyone who needs it.
You can download the plugin here: Download here
You can install it via "Settings -> Apps -> Manage -> Upload app"
More info about installing the app here
Please let me know if you are having any issues or questions