After postage info

How much approximately would it cost to send an envelope containing about 25 used stamps From the US to Australia (with tracking)?

Ok 6.30 sounds about right

I think it’s more like USD $23.95 if you want tracking. Note that First Class service has no tracking, but Priority International has tracking to certain countries (including Australia, as of July 2013) if you use Electronic Delivery Confirmation (which I think means buy postage online, not at the post office’s counter). Cheapest option seems to be a Priority Mail International flat-rate envelope.

If you don’t want tracking after all, you can probably send it for USD $1.10 via First Class International (assuming one ounce).