WHMCS, Web Host Manager Complete Solution, is traditionally a web host software that can sync with the server to automate account creation, billing, support tickets, etc., but the parameters used for most services and products, even virtual products, are so similar that the structure of this software allows it to be used for any number of applications. This is a licensed software and includes encrypted files, but does leave everything open that you need to customize the look and feel greatly.
Many tangible goods are just sold and then it's out of your hands as the merchant, but if you have a product or provide a service that requires more customer interaction and like the idea of having a support system built in, this software should be considered. Product creation is fairly flexible, not very flexible, but it gets the job done. Some things are lacking like cart views before checkout, but everything is tracked very well and is easily traversable for the end user too.
The Administrator features for this system are where the real weight is carried. There is a massive amount of information that can be collected and entered and is displayed in a nicely organized fashion and summed up very well. WHMCS provides many tools ranging from custom user permission settings, customer and product notes, invoicing, various payment modules, email piping for support, to more subtle but much appreciated features like To-Do lists.
More features can be viewed here:
WHMCS Feature List
Customization of styling is doable but takes a fair amount of work to really get it looking fully styled. There are so many options and various views and features, it's time consuming to style all of them, but it is doable. While there isn't a user friendly way to create front facing web pages, there is an announcement system that lets you post articles in blog format to keep your customers up to date on company happenings as well as a knowledgebase that allows for nested categories, restriction of individual articles to registered users only if so desired, search and recommended articles engine at the submit a support ticket page.
The system is very reliable and is used for a large number of the web hosting servers in the world. Other features like emailed database backups and once daily emailed cron activities reports makes keeping up with maintenance easy. Reliability and security are very important and WHMCS does a good job with both. The implementation of SSL certificates on the system is done well too forcing it to be used on pages containing or asking for sensitive information.
This might not be a perfect fit for all eCommerce situations, but it should definitely be considered before making a final decision.