Design a Sales Banner

I need a nice contemporary banner ad for my store pages (dimensions W. 400px X H.100px). My store primarily sells amino acid supplements like l-carnitine and beta-alanine for example. My store also doesn't target any one specific demographic. I want the banner to convey this message: If a customer buys any three (3) products in a single shopping cart transaction, they can receive a forth item at no cost.