Assign Shipping Class to Australia Post Shipping

Hi,
I’m wondering if this app has this capability.

My website sells some products that are bulky/heavy. I have assigned a shipping class to these. For these particular products, I would like to assign Australia post to generate a price. For all products, I would like to have a flat rate shipping fee instead.

Currently, I’m seeing the Australia Post pricing for ALL products and the flat rate fee is also visible. So far I can’t find a way to assign my Shipping Class to Australia Post.

I would appreciate any help! Thank you ??