am just wondering if it is necessary to purchase Digishop ($799) in order to use the plugin?? Does not make sense to me that the expensive software is free to use with WordPress.
I've not tried using http://wordpress.org/extend/plugins/digishop/ so I can't say specifically.
But generally speaking, yes it makes perfect sense. GPL software is free as in liberty not free as in free beer although most GPL software is both.
When talking about free software, it is best to avoid using terms like “give away” or “for free,” because those terms imply that the issue is about price, not freedom.
See http://www.gnu.org/philosophy/free-sw.html for more clarification.