Add a % discount tick box to my ASP.NET shopping cart checkout page.

I am using an ecommerce software package that is written in ASP.NET and uses MSQL database running on Windows servers. From what I gather Evolve ( the software name) is well documented throughout so hopefully it shouldn’t be too much of a chore to figure out how this all works. What I would like is simply a way that I can place a tick box form element on the cart page that when ticked will give the customer a 5 % discount. I want to be able to place this tick box my self and incorporate it into my design.