PNG has an efficient postal service and you can usually rely on your mail or parcels getting home, even if it takes quite a while. Note that there is no postal delivery in PNG, so everyone and every business has a PO Box.
International aerograms cost K2.50.
Letters or postcards up to 50g cost K6 to Australia and the Pacific, K8 everywhere else.
Package (5kg) costs K170 to Australia and K308 to the USA or Europe. You might’ve been feeling impetuous when you bought that 20kg skull rack, but you’ll be thinking long and hard about the cost of posting it home – K318 to Australia, K570 to either the UK or USA. All parcels are shipped via airmail.