I just use the prepaid satchels for things that will fit. Those that dont.. make sure you dont supply an approximation on your auction as i got caught out once when a nappy bag was sent the north queensland. I thought $10 would cover it.. $30 later i was a bit out of pocket for my mistake! So in that case just work it out on the Australia post website. You just need postcodes to and from, weight and measurements of the item.