Microcopy – Upgrade Once / Lifetime Upgrade / One-time Upgrade

I have a freemium iOS only mobile app that i'm currently working on.

The app give the user free limited access, and when the user requests access to a premium feature, I'm popping the option to upgrade.

When the upgrade screen pops, I'm trying to understand what is the best microcopy to use that will explain(self-explanatory) to the user that this is a one-time upgrade. I'm having hard time coming to a conclusion because English is not my native language.

What are your thoughts?