Security Review of e-Commerce Site

An e-commerce site has been produced for me to sell video clips on-line. The site includes a flash-based video cart that enables videos to be previewed and then purchased and autodownloaded. The flash cart itself is a bought-in product and has been integrated into a bespoke site. The site is due to be uploaded to a dedicated server in the near future. It is imperative that the site is as secure as possible and I am therefore looking for someone to assess the site and produce a list of potential vulnerabilities and a corresponding list of proposed solutions. I am looking for someone independent of the programmer who has completed the bespoke work as this will provide additional assurance. Once the solutions have been agreed, a separate agreement will be reached with the programmer to complete the work for me. An additional fee for completing this work will be agreed based upon the work that is required.