That "+" stands for the nifty integrated kickstand, and while the case is expensive, we did check out a sample, and have to say that it's really pretty nice. Agent 18 also makes the same case without a kickstand for $5 less, but we think that added feature helps distinguish the case--and it does come in handy.
So what makes this black case green? Well, Agent 18 promises that it uses one recycled plastic bottle in every case it makes. You'd think that would make the case cheaper, but alas, no.