Message on order printer if a certain Product is ordered

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I have spent hours searching the web and can not figure this out. 

So we have a product range that when ordered we plant a tree.

What i would like to do is have a line on the packing list (using order printer at the moment - will change if needed) that says

"thank you, your order has planted xx Trees"

If possible i would like it to only appear if that item is ordered which i can set the product up as a SKU. This way it doesn't say it on all packing lists.

I have this range in various products, so if I could also set each variant up with a SKU of TREE, then all products with that sku would add up and ideally it would print "you have planted 3 trees"

ANY help with this would be greatly appreciated!

please feel free to ask anything if i have missed any details required