Small business e-commerce package?

I develop web sites for small businesses in the UK.

I am looking for an e-commerce software package that supports a credit facility such as World-Pay etc., allows me to design the pages however I please, and then uses a flat file (such as a CSV file) to store the products and prices.

This way, once I have designed the framework of the site, my client would use MS Excel to create a CSV file of the products and prices for the web page to refer to.

The packages I have seen so far, like Shop Factory (Developer), don’t seem to allow the end-user to update the site without using the software.

Any suggestions?