Hi,
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 
and as soon as i’m getting into Zendesk will start testing.