PW,
If you want it free then Zen-cart (a descendant of OSC - Open Source Commerce) is the way to go.
If you want to pay I think Actinic is one of the better ones with all the bells and whistles.
You can see the work I'm doing using zen-cart at
www.itkit-uk.com - it has all the things you're looking for with extra modules to be had for free - like payment modules, postage calculator etc. See their site.
www.zen-cart.com. The template is also quite easy to modify.
Oh, did I mention zen-cart is a good one?!!
hth
Charles