The simplest way to accomplish this will be to use two add to cart buttons, one for the tires, one for the tires with a warranty.
A more complex way would be to add a warranty column to the cart, make sure to set the unique attribute. In the Add to Cart server behavior, set the warranty column to be updatable from a select list. To the select list that is added, add options for with and without warranty. Set the value for without warranty to 0 and with warranty to the cost of the warranty.
you then need to create calculations to affect the price of the products. On the Calculation tab. create a calculation called truePrice:
[Price] + [Warranty]
and change the total Price calculation:
[Price] * [Quantity]
To:
([Price] + [Warranty]) * [Quantity]
