Australia post ....again

Picture this.
I want to send something in an overnight bag. So I bubble wrap it and pick a bag.
Next the woman at the post office says my item is too heavy for the bag I picked.

I presumed that I would need a new bag, the next size up and pay the difference between the old bag and the new bigger bag, but.
NO.
The item can still go in the smaller bag, even though it is overweight and I just pay the difference between my phantom big bag and the smaller bag.

So what is the deal here. It seems to me I'm just paying for something I'm not getting. Go figure .