Nopcart was designed not to allow ordering fractional items!
Do a search of nopcart.js and change any parseInt(fields) to parseFloat(fields). (I found 6 places in the original.)
On line 259 in the AddToCart function there is a parseInt(strQUANTITY) that will need to be changed as well.
In the original nopcart.js the CKquantity and ChangeQuantity functions work with fractions, but if you run into wierd errors, check them as well.
Other than that, you should be able to use it without any problems.
Someone is using nopcart for fabric that is sold in fractions as well.