I missed this topic first time around, so I'll answer it now I guess
There are still a few markets out there requiring IRCs to be enclosed in the submission package for your reply. Fortunately these are becoming rarer! In most cases, the publication will tell you how many they want - 1 or 2.
Australia Post outlets sell IRCs over the counter. Get them at the same time as you log your submission package for postage costs (calculated by weight). Make sure you leave your submission pack open so you can insert your IRCs. Then seal and send.
There are even a few US markets out there who will ONLY accept US postage from a USPS outlet. That's way more difficult to comply with if you're submitting from down under!
As far as punctuation, grammar and spelling - if you intend on writing in a US market, then I would suggest altering your writing to suit THEIR preferencs - not your own. This way, the editor has no reason to pause and wonder if it's worth editing or asking you to rewrite. He/she is just considering the writing this way.
If you're in Australia, writing in the UK style that you already know means you could try aiming at the UK markets instead of the US to begin with? The exchange rate from UK pounds back to Aussie dollars is very favourable at the moment, so it's a great dollar return for your writing time.
I personally write for several international markets (I'm in Australia too) - and many editors still don't believe I'm not American. After several years of doing this, I can now happily switch between US and UK styles without too many editorial hiccups.
And - if you're going to play in international waters - make sure you have a GREAT relationship with your bank. Foreign exchange fees are HIDEOUSLY expensive down under unless you negotiate for a lower fee or switch to a bank that doesn't charge insane prices just to change over the currency.
Hope this helps
Lee