Okay, so last month I tried a little bit to get a response for this on GitHub and Twitter, but maybe I should have tried here first.
Anything along these lines seems to get pushed to the side and forgotten or ignored entirely. But I'd like to apply a bit more pressure on this.
Related purchases, nested purchases, microtransactions, etc. are pretty easy to create and sell on other platforms, and are extremely helpful in monetizing free games that wouldn't get players as a premium game.
But the only advice I can find for Itch is to create a new game for each DLC, which then has no real support afterwards. It's imaginable that creators could look up the purchases via API afterwards, but ultimately one would have to deal with the extra pages separately.
Ideally, we should be able to create an item/download with an individual price and product id on the same page as a game. How the download works or whether we go through the API to pick up receipts should be up to the creators.