Documentation quality and flexibility is excellent comparing to another non-php shopping cart I bought. I no time the store was up and running and shipping cost and taxes is a breeze. and also changing currency. This is what you need. I spent 2 days on this other shopping cart, with tons of complicated code and in the end it doesn't work, flexibility almost non-existent.
That's why I bought JQuery XML Shopping Cart by livelyworks. there is also freelance services (e.g to add functionality).