A studio machine that never touches the internet can still run these plugins. It takes one file and two minutes.
On the studio machine: open the plugin, choose Activate offline, and it writes a small request file to your desktop.
On any machine that is online: sign in here, open your plugins, and upload that request file. You get a licence file back.
Back on the studio machine: open the plugin and load the licence file. It is now activated, and it stays activated. It does not phone home, and it does not expire while the licence is valid.
If the machine is later refunded or the licence is moved, the offline licence stops at the end of its term rather than immediately, because there is nothing to tell it sooner. That term is written on the licence file.
If any of this goes wrong, write to support@pasttofutureplugins.com.