I've been tasked with relocating a small webshop to a new server running FC4.

The shop is running atop osCommerce, and FC4 has PHP5 installed.

osCommerce and PHP5 get along about as well as Steve Ballmer at a PLUG 
meeting.

I've been throwing random patches at osCommerce hoping it will work, but it 
just causes the symptoms to metamorphosize.  It claims it can't find 
directories I know are there, for example.

So I'm looking for a new cart/catalog engine.

It's a fairly minimal webshop-- only a half-dozen or so items, huge elaborate 
category breakdowns are probably not necessary.

It processes payment through authorize.net.

The software needs to be free; frankly, I doubt spending $100-500-1000 on a 
cart licence will be a good sell for the client.