Hi there,
Itch.io works on a Tier system. That means that if someone invests $5 on your project, they will have access to all files without a custom price, but also all files with a custom price of $5 or less.
As far as I’m aware, there is no way to change that, even with contacting support.
The only solution I can offer, is depending on what you are selling, you can create a new project page for each item. This is how it works with DLCs, soundtracks, or anything else you want to sell.
Hope that helps :)