Discount on a shopping cart that allows users that click 2 or more items to get 10% off whole purchase

I am working on a [login to view URL] shopping cart, I need a little project discount ( I am really decent with code but at this time and point I have to delegate this job I need a code in order to be integrate it on a shopify,com application that allows a client while shopping to receive 10% off whole purchase. A kind of sort of message that reads if you buy 2 or more items you will get 10% off of whole purchase then when that person click on second item will get a 10% off and so on if clicking 3,4 or any amount. Here is the website http://testedbatteries.com. Code it should be right on [login to view URL] while clients shopping. Please let me know how much and how long. Regards, Joe, Please contact me to I can give you access to account in order to see the code.