Ecommerce Printing Website

We are looking for an in detail eCommerce site for sign printing materials. Here is an example of what we would want it to look like [login to view URL], please refer to the tutorial page in order to get the general idea. When watching this tutorial please ignore "myImageZone", focus on the ordering process. Here are the different pages needed: Home About Us Contact eCommerce Functions/Product Page (in addition to the current product format, we would also like to add an accessory page that would simply be the product image and description along with price and quantity) Check out/Cart Most important is the coding that generates the pricing. The user will enter a size (ft / inches) which generates a price along with other variables. We do not want the standard Shopify drop-down list of size options. We are open to different design ideas, however we quite like the layout listed above. Upon checkout, the client will need to upload files directly to our already existing FTP. We will need this to generate an email notification letting us know an order was placed and that a file was uploaded. We will also need a shipping calculator that generates shipping costs based on supplied client location in addition to the weight of the product which can be supplied on a table, or as a "per square foot" number to the programmer. We are willing to use existing platforms/cart such as Shopify, etc... tailored to our needs and are open to any other suggestions.