I don't know about sending a T from OZ, but I have done it several times from here and cost me about $30 US to send a $20 T!!!
Can anyone inform me what the shipping tariff from OZ is?
If as I did you literally shave the envelope and tape to just on or under 250grams including the T-shirt, you can then sneak it in as an International Economy letter for just Au$8.
OK for kids and small women's' Ts.
The only other way is what nearly happened to yours Steve and given you a nice naval exposing cutoff like a Baywatch Chick!