Already owned items don't discount from Packs on Steam? A request to Reiza.

I initially bought Spa and Silverstone individually, isn't it possible for me to buy the
Premium Track Pack for the equivalent price of the remaining 4 tracks.
Let's say (with easier values to track), each track was 10$, and the 6 tracks in total were then 60$, but the Premium Pack was 40$. I bought 3 tracks, I would like to still be able buy the remaining 3 tracks for 20$ total, which would be the equivalent of the original price of the Premium Track Pack.
In this example, if I bought 3 tracks individually, and then payed the pack equivalent of the remaining 3 tracks, I would pay (3 * 10$ + 20$ =) 50$. I would still benefit from the Premium Track Pack but still pay more than the original Premium Track Pack itself, had I purchased all 6 tracks through the Premium Track Pack, for 40$.
Isn't this possible? If not, couldn't it be made possible? Seems fair practice between the consumer and the producer. And I believe Steam already implements this with Game Bundles. Please implement this feature on your store page for things like the Premium Track Pack, USA Pack and the Season Pass.
It would be appreciated.
Thanks for reading this!

 

There are games that have this feature implemented on steam. I think the option is there, it's just a matter of using it, I believe. Take this example for instance of the Assassin's Creed Franchise:



The already owned AC games are discounted from the bundle. I believe Steam allows this and it's up to Reiza to make use of it, but I could be totally wrong.

 

But couldn't then the Season Pass be a Bundle instead of an item, wouldn't that accomplish not only the same objective of it being an item that grants access to other items, while still covering this thread issue? Unless, a bundle can't cover unannounced/unreleased/unpublished content. But then, they could probably just add the corresponding items on Steam with one of these 2 ways:

1. A track to be yet announced but published on Steam, fixed price(same as others), to be released yet, would be up for pre-purchase. And then add it to the bundle. If Steam doesn't allow this, because it looks like shady practice, since they would be selling content that is 100% planned yet, they could op for option 2:
2. Announce all tracks from the get-go, and release each at it's own time. I believe this would allow to create and maintain a bundle with already released items and yet to be released.

Then again, I don't know the specifics or even policies involved and I could be wrong about this.