Every solution i come across seems to be a complete store with far too many options and too much functionality.

I can write my own cart to get the minimal functionality i need out of the frontend, im just trying to cut out the time writing a complex backend. What im really looking for is something that offers the backend functionality for a product catalog, order management, etc. while providing a smooth and consistent API to access the data for my completely custom frontend application.

The frontend will be in PHP so the API would have to be native PHP or some kind of web service interface i can create my own php models around. It would need to run on a LAMP stack and would preferably be written in PHP, Python, or Ruby just in case i need to customize the backend (she curently has ZenCart and hates it - says its far too complicated and sophisticated for her needs).

Does such a thing exist? Or what would be the next best thing?