Design the user experience and look/layout of a custom built shopping cart for an existing website

I am adding a shopping cart to my website and given the nature of the business we are building our own backend. I need someone to help me design the look and feel that fits with my existing website along with making the flow of the shopping experience in line with my competitors (but preferably better!). In all there will be about 4-7 pages, depending upon the design. The existing website is www.pacificpreciousmetals.com. We have some mockup pages of what the backend developers have created so far but these pages are just a placeholder for a better design. Those pages will be given to a few serious bidders and I expect some back and forth iterations of the design which I think can be completed in a 3-5 days. The user experience and design should be cutting edge.