You can create a project that is paid, and include demo files that can be downloaded for free. You can do that when uploading a file and ticking “This is a demo and can be downloaded for free”.

how can I use the Itch.io protection policy in my program?

Which policy are you referring to? The advice I mentioned above, combined with your account set up as “Collect by Itch.io, paid later” would mean that Itch.io will take care of sales for you, including refunds. Is that the policy you were referring to?