Hey there and thanks for getting in touch!
Good question. All the published data in the plugin is stored in your WordPress database, so it is as secure as the rest of your site. You can certainly install SSL and everything should be compatible there as well.
As far as payment transactions go, the plugin itself does not handle payments but is compatible with platforms that do.
Does this help answer your question? Please let me know. 🙂
Cheers!
Geoff