Add Co2 compensation fee in checkout

Hi guys!

I am going to implment a Co2 compensation fee feature in a Episerver Commerce project.

In checkout page, customer can choose to pay a optinal Co2 compensation fee based on the total weight of the products. 

If the customer choose to pay this fee, it needs to be a part of the total cost in purchase order.

What is the best way to implment this feature? Any suggestions?

Thanks,

ChiChing