I thing that the best solution is CS-Cart. It's really easy to use and customise. I've build my store on CS-Cart - http://www.qplayer.ru/ and now i'm customising it itself. I don't a good programmer or web-designer but after osCommerce i don't have difficultes for web-design integrate and customizing. And it's only $125. But i think now that all good php shopping carts such as - X-Cart, Squerell Cart, Cs-Cart and others are included all that you need. For more information - http://cs-cart.com/ or http://1shopping cart.com as a standart solution