We have a shop with two classes of product (p1 and p2). We want to sort by default to display by best_selling and then p2. (We could live with it the other way round). The p2 classification is a custom thing, we could easily convert it (create a custom short code?) or add an attribute? I guess this question has two parts, how to sort by a var and static value, and how best to add a value to be useful.