I think you might have got stung by the express post.  It's not just a bit more but ALOT more.

I put your mm into the aus post calculator, and Brisbane to Sydney was $28.70 parcel rate, and $82.80 express. Just as an example.

Parcel rate is not flat rate on the dimensions and weight of the parcel - it also factors in the distance that it is travelling.  Parcels within the same state up to 22kg are relatively cheap.

Hope that helps.