I have heard stories of parcels to the US going astray, but never letters. Hide the cash well and stick some childish stickers on the outside so it looks like a kid's penpal or something
Air mail to the US is a bit slow though (7-10 days, compared to about 4 days to the UK), but I'm sure it'll still be needed by then.
You could send it in a registered envelope if you want, but it'll cost you an extra $7.90...which on top of the bad exchange rate and currency exchange fees is getting a bit expensive.
The cheapest option might be to get a Western Union US$ money order from Australia Post, which will cost you $8.65. You'll save on currency exchange fees and registered post (it has to be deposited into a bank account like a cheque, so there's not much point in stealing it).
If you can get a group of bluelighters in your city to pool their donations, you could send it all in one money order and save on fees/postage. I don't know how much (if any) they have to pay to bank it over there, so sending a few money orders with as much $ as possible wuold be better than a whole heap of low denomination ones.
If anyone is going to a meetup this weekend, hold a whip-round (ask everyone when they're rolling, they'll be even more generous then
)