I tried asking this in the ecommerce forum and got no takers. I am (primarily) a php guy these days, so I'll try it here.

BACKGROUND:
I spent five years building an XML-based laboratory database for documenting wetlab experimental setups.
We made separation of GUI from backend processing a big priority. In the end we had HTML get/post forms, javascript/ajax versions and a pure Java Application front end to that database. And then, eventually, a Python/TK interface.

I'd like to find a shopping cart, preferably based on PHP/Mysql backend transaction processing, that advertises a well-known API, so I can chop away the default GUI and plug in my own home-rolled CMS. Which carts (OpenCart?) are the most cleanly written? Which would be the most convenient to modify, as above?