Hmm, on each file you upload, there is a tick box below that reads:
[ ] Hide this file and prevent it from being downloaded
That way you can hide any files in the project. However if there are any files left available, the project will still show up us purchasable. That way paid files can be hidden, and the browser version should still be available.
I haven’t tried this before, but I don’t see why it wouldn’t work. Hope it helps :)