Usually, if you click on "Postage and Payments", if the seller uses Australia Post it will say something like "Australia Post standard parcel", or something similar. If Australia Post is not mentioned, the seller usually uses another postal service. If this is the case, you can contact the seller before buying, explain your situation, and see if they are willing to use Australia Post. If not, maybe you can find another seller who uses Australia Post, if it is not a rare item.