There is no formal Zen Cart API, SOAP or otherwise. Some functions, such as displaying a products information page or putting it straight in a cart can be achieved by mimicing the URLs that Zen Cart generates (we use that to move back and forth between a flash front end and a Zen Cart back end at www.goateesaver.com), but there's no real documentation on this since Zen Cart is designed as an integrated package.
The database scheme can be found here.



