Create ecommerce website that blends seamlessly with original site

I need a shopping cart/e-commerce page for my already existing website http://www.stealthmachines.com. The design needs to blend seamlessly with my current site and look highly professional. I've been exploring zen cart and oscommerce... I'd like to use one of these two platforms (or something similar) to take advantage of all the features for future use. I want a very robust shopping cart that can handle the growth my company is expected to endure in the coming year. Design and back end needs to be highly organized and intuitive. ## Deliverables FULL BACKUP REQUIRED BEFORE MODIFYING ANYTHING ON MY SITE!!