They emailed me an invitation - wondering if that means someone actually looked at my stuff and thinks some of it would sell on business cards?
Pretty much. Naturally they are always looking for talent so they contact the artists that look profitable to them.
Still it requires a lot of design work to get a critical amount of card designs online.
I believe once you are approved your designs will go online instantly without review.
So as long as you play by the rules, your stuff is ready for purchase right away.