Masking relies on obsolete code and can cause problems on modern websites. I'd generally recommend only using Forwarding with 301 or 302 redirects (no masking).
The proper way to associate a domain with it's website is with either namservers or DNS records. With Wix a Premium Plan is required for these options.
I am not sure what you're suggesting. It is the 3rd party provider you're using that wants you to have an upgraded plan with them to be able to use it with a domain.