I just stumbled upon http://www.webtogs.co.uk/ an AJAX enabled e-commerce site with a wonderful balance between AJAX, usability and design.

One of the well balanced functions is when the user need to select color and size - sold out "combinations" is showed instantly as well as the estimated delivery.

Notice - the cursor delay when moving over the buttons is due to the screen capture software - when browsing the site without screen capture there is no delay.

Watch the video or check it out yourself on http://www.webtogs.co.uk/.