It seem's super cheap for planting trees. But it is not completely crazy. If I use the Lufthansa calculator to offset flights I roughly pay 20 Euro per 1000kg CO2 [1]. A litre of petrol produces roughly 2.3kg CO2. So roughly 450 litres produce 1000 kg CO2, meaning Shell can compensate CO2 for a quarter of the price of Lufthansa. It is suspicious, but with economies of scale it might be possible.

[1]https://www.lufthansa.com/am/en/offset-flight [2] https://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/oee/pdf/...