If mxtras.com is providing a specific version, it is possible they developed their own version of owncloud, so it runs on ReadyNAS. ownCloud currently only supports specific OSs, you can look at https://owncloud.org/install/#edition, they are for free of course. You can try to install a linux on the NAS, not sure if this is possible - than you can just install the native ownCloud. Otherwise, 5$ aren't much for some software, I think I would take the offer.
The other possibility is that mxtras.com is a service provider - then they offer their ownCloud to users who don't want to setup their own. Doesn't sound like what you want; I don't know though, because their website is just blank for me. Where did you find that offer?