Are there items (such as additional content, offers or currency) for sale that are used in the gameplay of your product?

Products cannot be directly purchased from within the game.
Under the Extras menu, players can see all the DLCs the game has and check their status (owned vs. not). Clicking on a given DLC opens the DLC's Epic store page in the player's browser, where they may purchase it.

Can your product be used without making any in-app purchases?

Yes.

Are in-app purchases refundable?

Products cannot be directly purchased from within the game, only through the Epic Games Store.
EPIC's standard refund policy is applicable to all DLC products.

What payment platform is used to facilitate these purchases? Do you use Epic Games’ platform or a 3rd-party (including your own) payment system?

Products cannot be directly purchased from within the game, only through the Epic Games Store using the payment methods EPIC offers there.