Cool initiative from an NZ company! 
We use them for all sorts: office supplies, kitchen supplies!
They are on our Wellington B Corp community shared list of preferred suppliers!
From them:
Now we up the game, and plant a tree for every EcoWarehouse-Branded Product purchased from our website. There is no action required from your end and we will bear the cost. Said that, if you like the idea, please spread the word far and wide and invite your family and friends to shop on our website for all your Eco-Friendly products. Our range now includes over 440 products to choose from, including a range of environmentally friendly Xmas ideas.