Is it possible to add substract function by add to cart?

[RobbertG]

Hi all,

I want to archieve substract function in the add to cart module. It's like this, you add 40 products in the cart, then you wish to remove 10, you just put 10 in the quantity box and press on a substract button and the quantity is recaclulated to 30.

Anyone knows if this is possible?

[Maxim Pishnyak]

Could you give an url or a picture for prototype of this?