change cart tracking to take account of tax & special offer pricing

[kevinb456]

The cart tracking uses the main product 'price' attribute, not taking into account special offers / taxes. (however ecommerce orders do correctly take account of specials & taxes).

Would be good to change in future - not sure if there's a simple way of simply finding the advertised price (on product page). Product page uses several PHP tests to work out actual price dynamically. I dont think its ever stored anywhere statically as it can depend on several things, so I may just have to replicate the same code that the catalog/controller/product.php page uses?