I just came across this post so thought I would my 2 cents. I am the founder of Sprout, the #1 tree planting app in the Shopify app store. https://apps.shopify.com/tree-planting
Sprout allows you to plant for every $x dollars. So you can choose to plant 1 tree for every $10 spent. The app also has a cart badge that is dynamic and auto updates to show how many trees are being planted. i.e. If its $115.80, it will show 11 trees planted. We currently don’t have the option to show “Spend $4.20 more to plant 1 more tree” but that’s something we can build with you if you were interested in trying our app
Also, our app also has the feature to allow your customers to donate or top-up to plant more trees e.g. Plant 2 trees for $2 to help continue growing your forest.
Hope it helps if you are still looking around for a solution