Think of an eCommerce system as just extra functionality added to our Base Build Websites, don't know what that is? read about it here: Base Build Website
Our standard eCommerce build generally includes:
All products and information would be edited by you the client (tuition can be supplied). Any site layout & template modifications are usually done by the designer. Once you start to get used to the workings of the website, there will be no maintenance cost until you decide to upgrade the look of your site or add extra functionality.
In saying that however it is a good idea to take out a Support Package for those little surprises that occur in life and computers.