Hello,
The strange this is that when items are grouped with the correct dimensions of my products the postage price never changes (i.E $18 for 1 or 5 of the same product is always the same) but the order can go through, however the postage cost is a lot higher than is calculated by the Aus Post module.
I've fixed some part of the Australia Post code, so a solution can be to use that custom version to see if it fix your issue.
Our products are collected by australia post dailey from phone orders and have never had an issue with them for any of our products. So the calculations from the aus post module are not working correctly
The Australia Post shipping plugin is only displaying what the Australia Post platform is returning.