Some people believe you are better off paying for shopping cart software due to the support provided. However with popular carts such as Zen Cart, you can find extensive help in the community forums. Because it is so popular, a wide range of templates are made available by template sites. Furthermore, an extensive range of add-on modules are made available to download by other developers, enabling you to fully customize the store to your suit own requirements. From personal experience, I have found Zen Cart easier to modify than osCommerce.