Please forgive this if this sounds in anyway condescending but I am genuinely confused!
The point here is you do not purchase a download. You purchase an Account.
Unfortunately most software stores are setup to sell you a download.
So, if you BUY a game on Oculus/Steam/Epic or even the frontier store, then you get access to a download and behind the scenes you've also gained access to an account.
Think about how you can switch accounts in the login screen of the FDev launcher, you don't need to download the game multiple times, you can just flip from Cmdr A to Cmdr B.
What you want to do now is to take your account that you purchased via the Frontier site, to a download from Oculus. FDev give you the ability to do that with a free key that lets Oculus know that you have already paid FDev, so you get the download (with Oculus enhancements) for free. Once you have downloaded you can login your normal Account and carry on in VR.
i.e. One payment = One account
but also One Payment + One free Key = Two Downloads