You will not be able to use eCart to make the association between the cushions and products, this would be up to you to do either by making this association in a table on your db or by manually associating different cushions with different products. Based on what you are describing I'm not sure that eCart would make this any easier for you since it would require you to change over the cart completely in order to make use of it. Like you noted, it is a long path for a quick solution. However there are other advantages of updating the cart in the site to make use of eCart, you could then easily edit all of the 'add to cart' and checkout server behaviors without having to edit any code directly. If this is a long term project that might need more customizing or updating in the future it might be a good idea to think about incorporating eCart into the site.
