Hi there,
The individually priced files work in a tier system. That means is a person pays £10 to your project, they will have access to your base project plus all files with an individual price of £10 or less.
The system wasn’t designed to provide DLC, as you’ve discovered by the problem you’re having. A good idea is to have DLC on separate project pages and link them to the base game, but there’s no hard rule for it. I understand having each character in its own project page might be a bit too much work, it’s up to you to find a system that works for you.
Hope that helps, Cheers