Shopping cart is not updated after add to cart when you select an attributes
Here is where I would do like the big stores, and add a separate line item (product) for the warrantee add-on where you you can have two variations of it, one for $100 add-on value, the other for $200 add-on and then in the product display page you can have a customer select which option if any to select, which will then be itemized in the cart by itself with its own description. If you want you can also have a product display page or just a page for the two to them. It would be a product type by itself, with variations as opposed to color terms describing variations of a cell phone.
And it might be easier on cart space.
And Warrantee variations are products that can be related by name.
You end up with less sku products: The cell phone product color variations, and the Warrantee two variations.