Hi joelman
Welcome to the forum ... and sorry for a late reply

I'm also from Melbourne.
First of all, you're not breaking any posting rules that I am aware of.
Secondly, this is how you'll go about your request:
1. Write your letter
2. Get 2 envelopes. One for your SAE and one to address to Zach
3. If you are sending something to sign, send a SAE large enough to fit your items. If you are not sending anything, my suggestion is to send a large A4 envelope. Address this envelope to you.
4. Address your other envelope to Zach (this envelope can be any size, provided it fits your letter, SAE (and photos/items to be signed if applicable)
5. Purchase the correct stamps and IRC's. In Australia, the current postage price to send overseas is $1.95 (for 50g). IRC's cost $2.70 and you will require 2 if you send a large A4 envelope. Smaller, standard envelopes require 1 IRC.
6. Put your IRC with your request and SAE (don't put it in your SAE as it may not be seen) in your envelope addressed to Zach and seal it up.
7. Stick the relevant amount of stamps on your envelope addressed to Zach, add an AIR MAIL sticker to (or write on) the envelope
8. Post away
It's a good tip to write "DO NOT BEND" on your envelopes, as it may reduce the chance of your photo coming back not bent.
Also, to clarify ... a SASE (or SSAE) is a self addressed stamped envelope. You can only send one of those overseas if you have the relevant country's stamps you're sending to (e.g. US). Otherwise you will need to send a SAE (self addressed envelope) and IRC(s)
nowandagain [rockon]