From the store admin menu click the manage categories button and enter a keyword in the "URL Keyword" field and save. If you have a url keyword for each of your categories then the cart will rewrite the url with the keyword in the url and give you a unique url for each category that you then can link to from another page in your portal. If you dont have a url keyword defined for a category the cart just does an inter module communcation to the buynow module to filter the products so there is no postback.
-Scott