There is no option to set the price as $0 on the price selection page.
Hey @animo-mocap! Welcome back!
Generally you can not set a product to free unless it is a plugin. Some products do get moved to free (post-publishing) under special circumstances. If you submit a support ticket to the Unreal Marketplace you may be able to get more clarification on how that process works.
Hope the above is the solution you are looking for!