Australia Post Shipping Calculation - not working for my type of product

Hi
I am revisiting using the Aussie Post Plugin mainly because some clients want packages to be sent to a PO Box - which Sendle Shipping won't do. However, I can't use Aussie Post for one of my main products. I am looking for a workaround.

I have a product that is effectively like a tray. It is 45cm x 45cm x 5cm deep. If I order one of these the Aussie Post calculator is correct.
However, these are normally ordered in multiples. The way hikashop works is that it takes the 5cm deep measurement and and multiplies it by the number of products ordered. So if there are 6 of the same product being ordered, the overall measurement will be 45cm x 45cm x 30cm. BUT, each of the products actually sits inside the other so the first product is 5cm deep, when you add a second product, it adds only 1cm so is 6cm deep so for 6 products, they are actually only 10cm deep.

There is a lot of difference in shipping cost as a result.

To add to the complexity, I have a specific box size which is slightly bigger obviously than the product so is 48cm x 48cm but the depth is 11cm. This allows me to send anywhere up to 9 of the same product so shipping volume is the same, but weight will be slightly different.

I just can't find a way to replicate this at all - and can't think of a workaround where I am not either over charging customers or losing money each time I ship.

It would be good if I could set a weight and volume for specific quantities ordered but doesn't look like this possible.

Any ideas?

Ideally, I would like to be able to