Change Shopping cart in OSCommerce/ Add product

When people want cushions made they enter the dimensions in the website OScommerce figures out how many square feet and charges them per square foot. That value is then carried into the shopping cart but they can change the quantity which alot of people think they have to because they think the quanitity is how many cushions rather than how many square feet so I need them not to be able to change the quantity for cushions once they get to the shopping cart. I need other products to still have the option to change the quantity. I also need a product added that is has the exact same attributes ans charge per square foot as another product. So basically I need the product to be duplicated with a different name and desription.