Most Customizable Shopping Cart Program?

I am looking to implement a new, highly customizable shopping cart for a few ecommerce websites. We are currently using a 3rd party one we link into that is erratic, inflexible on the design, and difficult to make changes on in a timely manner. It is preferred to work with one that is PHP / MySQL based.
The following are the requirements we are looking to meet
o Convert .psd layouts we produce into shopping cart and web page templates or themes

o Ability to use our collection of zip codes to provide different shipping and delivery costs as well as different shipping and delivery options to customers based on specifications we set for that particular product

o Support available

o Use CSV files to add / remove / change quantity / change stock / change product specifications

o Emailing of order confirmation, tracking confirmation

o Possibility of working off of a database we would create

The ability to customize the look and feel and adding functionality like the zip check is crucial. I am only familiar with so many of the shopping cart software and am open to the possibility of bringing in programmers to develop our own or do work on an existing one