Personally, I have looked into CubeCart; this is a decent application and doesn't cost much. The front end is fully templated so the appearance can be customised to great extent. osCommerce is functionally very usable too, but customising it is a complete nightmare. If you can use it out-of-the-box and don't care about web standards this one is worth considering as well.
What I like about CubeCart is that you can install it for free, configure
everything, make it work and if you decide you really like it and want to
eliminate the "CubeCart" references, you just pay them and go ahead and
remove all references ... very simple and easy to customize.
I also noticed a lot of people are offering fairly cheap gateways to various
credit processors that CubeCart doesn't support, although they do support
most of the popular ones.
I personally favoured it, but as mlseim indicated you can try it out for free and see if it meets your requirements.
I haven't looked indepth at more expensive packages, but I know of at least one more expensive one that gets very good rep as well; I just forgot the name...
CubeCart here, too. You can freely use it forever, or pay $89 to remove "CubeCart" (which isn't even obtrusive), so it is a great deal. The XHTML and CSS files are straightforward to work with, plus they have an active forum that is also open to all users, free or paid.
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SIM --- Simple Integration Method, where the user leaves your
unsecure site and enters a secure site (like PayPal). It's there that
they enter their credit card, then they are returned back to your site.
AIM --- Advanced Integration Method, where the user stays on your
site, which needs to have SSL/HTTPS for the credit card entry and
also a secure server for the database, where information is stored.
Your scripts sends the information to your credit merchant directly
using a bunch of keys.
A credit card merchant like "authorize.net" allows either SIM or AIM,
it's your choice. PayPal is pretty much SIM (as I'm aware of).