Complete the Javascript to make a price calculation table work.

I have an on-line price calculator table which runs inside a Wordpress website, simply embedded the [login to view URL] with a javascript script running from the [login to view URL] which handles the customer selections. I've set up a test instance of the site here: [login to view URL] With the above site in the "our prices" section you can see the basic function if you select a small car, wash n' vac and an extra 'screen wash', you can see how this is meant to work. The price updates and if you click book then a very basic form appears in order to send the selection to the booking to the admin email address. The problems begin here: The "extras" are conditional. For example if you select a Wash n' wax you cannot request a "Pet Hair Removal" as an extra and if a user tries to do this they should be shown a message requesting they change to a Wash n' Vac and then reselect "Pet Hair Removal" as their extra. If a customer changes their car size, their existing selections should be cleared to avoid a price exploit. Three additional extras, "tar removal", "back to black" and "odor removal" have been added, although they do function slightly as intended the conditional functionality seems to not be working so some of them cannot be selected at all, others should be exclusive to a wash n' vac but can be selected from a wash n' wax. A feature to enter a voucher code needs to be added as a text field. If the value the user types in this field matches a valid voucher code it will will apply a variable discount. Codes need to be changeable either in the Wordpress backend or otherwise easily changed. I have attached the [login to view URL] in question for you to have a look through, bare in mind the code is referencing wordpress custom fields, I can provide a Wordpress login if needed and this is just a test copy of the master site so you don't need to worry about breaking it. Just to clear up any possible communication issues could you please include a quick summary of how you understand this task and what you think I want you to do for me.