ECWID Shopping cart modification

I need to customize ECWID shopping cart for a wood flooring online store. The wood is sold by price per meter and a box can have different m2 amount therefore I am looking for a calculator where the customer would enter the amount in m2 and ECWID would calculate how many boxes they need. Here is an example of what I want [login to view URL] Can someone please help? I would also be open to sugesstions of different shopping carts that would enable me this feature. The reason I would like ECWID to have this option is that I can use this shopping cart on facebook. Many thanks