Sorry, there isn't actually a way to search for calculated postage listings only, you can really only confirm if they're using Australia Post services, and by seeing if the postage rates change when you enter a different postcode (in the postage and payments tab, there's an option to change the postcode and get a quote for P&H to that area, I switched mine from an SA postcode to a WA postcode to check - you just have to remember to change it back again otherwise you'll keep getting quotes to the other postcode).

When I double checked, I looked in the book section because a lot of sellers use calculated there, but I would assume anyone posting the kind of items that have variable postage costs depending on the buyer's location (due to size / weight of packages), would likely use calculated, so the glitch would be relatively widespread in any category that applies to, unfortunately.