This is happening because you are using “Individually priced files” to set your product price. Please read our guide here:
https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced-files
Use individually priced files for quick bonus content, not to set the price of your content. Use the minimum price on your project to set the price of your product, as this will allow you to change the price at a later time without affecting ownership.
We strongly advise against using individually priced files. If you have a free file for a page that is otherwise paid, select the “Demo” option, and set the product price of the page.