can someone help me with please?

I'm making a shop with products like tomatoes, potatoes, ... the problem is that i want that the customers can purchase for example 2.5 kilo tomatoes but i don't knwo how to do this. i'm working in a template that i've bought and which only allows to purchase 1,2,... kilo of a product.

Does someone know how to solve this ???

http://www.cok-o.be/winkel

Kind regards
pascal