Javascript developer needed to create voucher redemption

I currently have a website which is all static HTML. I need a way of applying a voucher code to checkout. I can not have the links changed as these are highly ranked by Google and don't want to disturb this. What would be brilliant, is if a page is created specially for voucher codes to be entered e.g [login to view URL] Here the user would enter their voucher code and then the website would check if this code is valid in the database. This then sets up some sort of cookie so when a user goes to buy a product, the given discount is applied. I need an admin area for managing my voucher codes e.g. the code itself, if it has been used and how much discount the code gives. Each product page can be set to accept or not accept the discount. I think this bit needs to be done in Javascript since it needs to be client side technology. Obviously the voucher code page can be PHP. I don't know how much is possible given a static HTML website, so any suggestions are welcome. Low budget. Can provide website link on request.