Hi Holly,
just a quick answer. I've been searching the market for RMS integrated e-commerce solution and it's true that the only two solutions that You can get feedback about, and they are Cosmos and NitroSell. I know a little bit about Kosmos, mainly from what I have found on the net, but I must say that my experiance with NitroSell have been more then good. The support we are getting from them is just great. Really, it is a pity I can't have more time for NitroSell Commerce (leading other projects, etc...).
The software is very solid, have not found bugs or problems, easy to use and very user friendly. They have local support, just as my company does for our final clients, so don't worry about that. It's true that there is a part of the code that is "not available for editing", but You can always talk to NitroSell, they are doing Customizations also. But on the other side, I have found many ways You can edit or customize the web (html, script, css, the standard options, etc, etc...). Also, the way the system works, it's imposible to be attacked by hackers, it's integrated with all google/froogle/constant contacts, your RMS SO database is safe since what the the final customer is seeing is only a "view" of the data you want to show, etc, etc...
Feel free to contact me if You need more info. Hope I'm helping...
Regards, A.