Sorry, I may not have explained it properly, but the calculated postage does it all for you.

It uses the dimensions and weight you enter and automatically calculates the postage to any destination in Australia based on Australia Post's rates and using your postcode plus the buyer's postcode. If set up correctly, when you look at the listing, the P&H shown will be the cost from / to your postcode, but if I looked at the same listing, the P&H cost would be different because it will be calculated from your postcode to mine.

I hope that makes more sense.