MAJOR Issues with Products and Custom Fields

[freshweb]

Our products have different prices for different sizes available.  We can get the drop downs to appear with the different options, but when you select a size option and go to the checkout the price is added to the main product price instead of simply changing the price to the price of the child product.  We dont want to have to manually calculate the difference for each product as they all have their own prices.  hope this makes sense.

  Please view the product example here : http://175.107.146.153/~kitesurf/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=30&virtuemart_category_id=18&Itemid=566

Is this possible.  This was really easy to do with the old version of Virtuemart but I have wasted a whole day on this and are at my witts end.  We might have to look for another shopping cart facility