Hi,
As you can see in your shipping methods, the two methods have weight restrictions, the second one require 1Kg to be available.
And because the AUPost plugin does not return any method, HikaShop notice the user that it need more weight to have an available shipping method (the "Manual shipping" one).
So, now we have to understand why the AUPost plugin does not return any shipping method.
And regarding the error message you got (the divide by zero on line 3082), we can see that you have a product with an empty weight.
We will improve HikaShop in order to handle that specific cases but the fact that you have a product with an empty weight in your cart is not normal.
The AUPost plugin require weight on the products in order to get the prices for the order ; to be shipped, a product require a weight.
Please check that all of your product right have a weight configured.
Regards,