Our website is having problems with sub-categories.

We have a WordPress site that uses the Shopp plugin for eCommerce. We have multiple header categories. We need to able to assign and sort based on sub-categories as well. We don't want these sub-categories to appear on the Header. For instance, we have a line of greeting cards. We'd like to be able to view only the Holiday cards, etc.

We would also like to be able to sort the header categories in any order that we want them to appear. Currently, they are alphabetical.
Thanks!