mcotten - 7:14 pm on Jun 12, 2013 (gmt 0) [edited by: lorax at 5:24 pm (utc) on Aug 27, 2013]
Ecommerce service providers are a great choice for your client because they include the shopping cart software, web hosting and support in one monthly fee. Volusion, Shopify, Bigcommerce and Yahoo! Stores are the leaders here.
Although they include the shopping cart and hosting, I would not consider them "pre-built" because even when using a template there is still plenty of work to be done with configuring the store, adding products, and setting up shipping and payment methods. These providers do not provide any setup services.
Volusion and Shopify offer premium templates at a cost and Volusion also offers a custom design service. Volusion customization is limited however and I don't like that Shopify charges a transaction fee. Yahoo Stores! is considered outdated. My preference is Bigcommerce. I use it to design and setup websites for my clients as I found them to be best suited to my clients needs and I can have full control over the design.
If you know HTML you can create or edit the templates but there is a bit of a learning curve. I recommend reviewing the available templates and try making some basic modifications to customize it for your client. All the providers have a free trial so you can try them out and see which you are most comfortable with.
Here is a link if you would like to try Bigcommerce
[edit reason] no links pls [/edit]
[edited by: lorax at 5:24 pm (utc) on Aug 27, 2013]