Thanks for the info on Yahoo store. My assumption was based on the relative uniformity of the sites I see on it. Also, you can't actually modify any code, right? So if you wanted a special feature, like a gift registry that would give discounts to people who buy from it or a special offer for customers who meet certain conditions, you can't really do it yourself.
Maybe you could update your answer with some information on the flexibility and limitations of the Yahoo option?