Domestic Express Post
$15.00 guarantees next business day delivery if a delivery address is within the Express Post network on a business day (Monday to Friday). This covers 80% of Australian business addresses, private addresses and Post Office Boxes. The service operates between all capital cities (except Darwin and in Perth CBD only) and some major centres. If the address is outside Express Post network, it will be sent via the fastest possible transport links, but it won't be delivered by the next day.
Although Australia Post Airmail Service guarantees reliable delivery to over 200 countries, customs procedures in other countries have been known to delay the delivery of packages.