No one is suggesting that you are listing with $45 postage, but part of the reason the fees on postage came in was because a lot of sellers were. Sellers who listed with free postage were paying fees on the entire amount. Other sellers would list for 1c with $99.99 postage. They would only be paying fees on the 1c, which is basically impossible. They had to be stopped. And they were, for the most part. Some sellers still try it on, and then get a big surprise when they realise that the fees also extend to the postage cost.

As for GST, you are only paying GST on the FEES for the postage, not the postage amount. So, if you charge $9 for postage and the fees are 90c, you're only paying GST on the 90c (I know fees are a little higher than that, I was just trying to simplify it).