To amend a ecommerce wordpress plugin by coding a small change in the calculation5

I am re-submitting this request due to a slight change in needs and also i am being more specific about my requirements. I have a site for artficial grass and there are just 5 different types of the grass - each type comes in 2m and 4 m widths.

The problem When I set up any ecommerce plugin it will allow me to set the different widths which is fine. They can then order a qty of metres of that width. (for example 5 x 2m widths. What they cannot do is order 2.5 x 2m widths - because when they place this in the cart - the calculation defaults to 2 - so it only calculates whole numbers.

What I need is that they can select for example 5.2metres of Verwood grass in a 2 metre width (verwood is 27.98 per metre length of a 2 metre width) so 5.2 should calculate to in the cart to 145.49 (two decimal places is fine) This is just a sample calculation

what i need is for someone to amend the code within the ecommerce plugin so that it calculates the part amounts ie 2.2
I am not that bothered about the ecommerce package used within reason so am open to suggestions for the most appropriate one for amending the code to suit.

Please only bid if this is something you can do - i do not want a full bespoke package building for me. The budget is tight, but i am likely to put more work your way in the future.

Please put a proposal togther based upon my requirement. Thanks