e shopping

I'm new to doing the webmaster stuff for people other than myself. I'm starting to get a few clients now and although nobody has yet asked for a e-shopping/catalog/cart/ecommerce script yet, I was hoping to get a little advice on which one I should recommend if needed.

I just started looking at at free, open-source scripts. Easiest for me to install would be interchange and agora because they can be autoinstalled with one button by cpanel. But the interchange website states that it is a big learning curve and meant for big inventories (my future clients would likely be small catalogs, if anything at all). Agora looked easier but less snazzy. And I also came across oscommerce (I think that's what it was called) and that looked really good but I'd need to install it from scratch.

So what would all y'all recommend for an easy to set-up free sales type app that would also be easy for my know-nothing clients to use too. Bonus to which ones may be easiest to squeeze inside an existing site template layout.

Thanks!

P.S. Their business would need to be able to process credit cards themselves, right? Or will these programs take care of everything.