I visited the link and added the item to the cart then proceeded to checkout and did not see an option about the cushions. In general this type of thing is not too difficult with eCart if you have an understanding of data involved and how you would like it to work.
If you have the cushions as another item it would be very easy, you can just add them to the cart and the price fo the cushions would be included in the subtotal. If you would like to make it all the same product it can be a little more involved. If you can describe a little more about how exactly you would like it to work I can tell you about what would be involved and how much skill would be required to achieve the result.
