I can imagine a variant of this card that tries to tackle the point you bring up. Not all purchases are equal. Some of them have negative carbon footprints. The tree planting offsets the footprint for every purchase. These things are not easy to estimate exactly but since there are probably order of magnitude differences in carbon footprints for different products there should be useful information about the relative impact of a purchase to display to the consumer and to demonstrate the overall impact of the project.