There is much better documentation for OsCommerce on the OsDox site: click here
Don't ever mention it on the official OsCommerce site though, they don't approve of OsDox.
I use a PHP cart called ... um ... PHPCart! click here Secure Trading is one of the supported payment gateways. It's very cheap but not as sophisticated as either more expensive software or OsCommerce. It doesn't use a database.
(I just clicked "post response" and it ummed and arred for a while then did nothing. Last time that happened I clicked "post response" again ... and my message appeared twice. I've just refreshed the page and my post hasn't appeared yet so I'm going to try again.)