Add tofixed and promocode to checkout page

Hi, I just need some quick work on one checkout page. This is will take less than 1 hour to do. On my checkout page, I need to add a tofixed code because the floating number goes banana when adding features for the order, which returns an error when trying to access paypal. And I also need to implement a box where a user can put his discount code to get a discount with the item purchased. The box is already designed. You will need to use javascript so user can show or hide box this box. Also when code applied, a text next to the box will say that the code entered was valid or not valid. Also make sure that the code can only be entered ONCE. The user must not be able to apply the same code ten times. Just Once! Please make sure that discount code is only applied to total amount!