heidi, last year I bought some cash from Aus Post and I got the best rate that I could find at the time. Yes, I did have to order the cash in as they don't keep USD on hand. But what they did was took my money and locked it in at that day's rate. I got given a receipt with the exchange value etc and I think it took 24 hrs to get it in.
It was pretty easy and I didn't mind waiting for the cash as it was locked in at the rate I wanted.
I would def use Aus Post again
xxx