I use Cactushop http://www.cactushop.com/ - A couple of weeks ago V6 was released - this is fully css and XHTML strict and has the features you need. A couple of sites I have made using V5.1 are http://www.johnleachpottery.co.uk/ and http://www.mtmc.co.uk/

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Changing Constants Ltd - 'the only thing constant is change' - affordable e-commerce website design