As the copyright belongs to the artist, which shows that you care about your artists and is a great thing to promote, I think the wording on the reverse should reflect that – something like:
© “Artist’s name”, designed exclusively for Wuzci/produced by Wuzci
Also, I think you should push the fact that the company cares about the environment. Your website says “We use recycled paper to create our cards and envelopes, print with non-toxic inks, deliver with minimal emissions and even plant trees as part of our Green Tree Promise.” Why not shout about this on the backs of the cards, too?