I've installed osCommerce several times for freelance jobs (and just recently for myself to see what had changed since the last time I did anything with it) and really like it. It's got a good solid database structure in case you want/need to customize things, some decent basic reporting, pretty easy to manage catalog (although it looks like the initial inventory set-up is extremely manual; I could be wrong, I haven't looked very hard yet).
Poking around, it looks like the Authorize.net integration is pretty painless, and there's PayPal integration also. It's pretty much plug-and-play so long as you already have your account set up.
osCommerce is open source, too, so there's always someone somewhere coming up with add-ons like gift certificates, etc.
Feel free to pretend shop (it's not hooked up to any actual payment providers, so "buy" all you want) at http://spackletsunami.com/teststore/catalog/ , and I'd be happy to give a couple of people admin access to the catalog if you want to poke around there as well. (I also promise I won't sell any email addresses used to register or whatever.) What you see there is pretty much what it looks like out of the box with no customization or style sheet wrangling. That took me about 15 minutes and not a ton of technical know-how to install - it's a really spiffy package, and it's come a long way from a couple of years ago.
Feel free to use the "contact us" form or email me via the address in my profile if you've got any questions or would like to poke around in the admin part.
