Can't play local co-op because adding missing DLC Repentance is impossible, HELP

[Elimeliilemilez]

We're trying to play local co-op and it's not possible because NOT A SINGLE PLACE on Steam allows us to add Repentance to the game, but Repentance+ won't work without it. We have Afterbirth, Afterbirth+, and Repentance+ downloaded - literally just missing the base Repentance which CANNOT BE ADDED.

How are we supposed to fix this???

The developer has set this up in an extremely convoluted way and it's completely put us off on trying this game, so asking here is a last ditch attempt in the offchance someone somewhere knows what you're supposed to do about this.

Clicking "Add to cart" on the Repentance, Afterbirth, and Afterbirth+ bundle at this URL https://store.steampowered.com/app/1426300/The_Binding_of_Isaac_Repentance/ does NOT work, it throws an error 8 "There was a problem updating your cart.".

And on the Repentance+ page, you can download it, but you can't just grab base Repentance, and there's no solution for when you already have the other 3.

This is ridiculous and feels like bashing head against a brick wall. Just give us a "download" button somewhere, please?

[Elimeliilemilez]

https://imgur.com/a/aBCF0fU
screenshots

[Nazemec]

Repentance can only be bought through the Complete Bundle. It's that way to prevent people from getting it without all its requirements.
https://store.steampowered.com/bundle/2405/The_Binding_of_Isaac_Rebirth_Complete_Bundle/

As for any other bundle, Steam won't make you pay for things you already own, you'll only pay for the missing parts. In your case, that means you'll only pay for Repentance (which isn't, never was, and never will be free).

Why this logic wasn't implemented for Repentance+ immediately (and still isn't) is a mystery to me.