New Functionality to WordPress signup and checkout form

I need to add a new functionality to be added in WordPress signup and checkout form. When a user adds a product to the cart and goes to the signup page, there should be a new field where user can enter his zip code and after entering zip code, a list of service stations appears under. Then user can select their desired service station. That quite simple. Most of the work is already [login to view URL] is the website : [login to view URL] .