It looks like they just buy tradeable renewable energy credits. My electric company does that and it costs less than 1.5c/kWh. (I know, because we buy them.)
When renewable energy is created it's basically divided into the actual energy and these carbon credits, because when it goes into the grid it's all just electricity. Then the renewable producer can sell the credits separately, and other parties can purchase the credits and then market that energy as renewable. So I'm getting coal electricity with as many kWhs of wind credits as I actually use, and paying a little more than I would for coal electricity without the credits. If that's worth it to you, you should see if you can get those credits for less than the prices that company charges.