Basic Javascript coding

We have a simple web form for selling bus passes. A user selects the number of bus passes they want and the form totals the amount and allows them to submit it via a script that e-mails the form's contents to us for processing. We wish to implement a coupon code box that allows a user to enter a single, predetermined coupon code. If the code is entered correctly, the total box will discount a particular type of bus pass by 25%. Only the javascript to make the coupon code work is required for this project - everything else will be provided before you begin work. We understand that using this technique will result in the coupon code being able to be found in the html code for the page - this is not a concern.