The product details page supports linking directly to the product details page by passing the product id and the variant id in the querystring.
For example:
http://www.yoursite.com/productdetails/Default.aspx?productid=1&variantid=4
When the product id and variant id are passed in the querystring the products details page will be loaded with the product and variant information associated with the product id and variant id passed in the querystring.