Some of the add-ons are part of the reward tiers, while others are standalone items that can be bought separately and do not come in any reward tier. The digital add-ons include Inner Circle Videos and the digital art book (available at the $50 reward tier, but can also be purchased for $10 each), additional copies of the game, developer's commentary, a designer's notebook and the developer's unreleased novel.
The new physical rewards include bookmarks, postcards and pins.
To purchase the items à la carte, backers simply need to change the amount of money they are pledging to account for the purchase price of add-ons, without changing the reward tier.
The Kickstarter campaign has raised $54,256 of a $99,000 goal at the time of writing. Our interview with the developer can be read here.