HELP - Edit cart count

[VBQX]

Hello everyone, We are looking for a solution to withdraw one item from the cart count. (When 2 items, only display "Cart (1)")

We currently have an app installed (One Tree Planted), that automatically adds an item (free tree to be planted) in cart. This means that when a client arrives on our website, his default cart already has 1 item in it (tree).

At the moment we managed to hide this item in the cart, and only display it at checkout.
Now we just need to subtract -1 to each cart count, so when a client arrives on our website, cart is empty by default.

We believe this should be fixed with some Javascript.

Thanks a lot for your precious help!

Our store url is : http://atelier-doravante.myshopify.com
Password : Dessein19129410!