Hi Kirstie
One IRC will cover a standard letter to anywhere in the world.
I know that when I get letters back from the US after sending 1 IRC, the stamp is only worth 84 cents in postage (I only send out 6x4's to sign anyway) :x
Since you do live in Canada, I know that it's cheaper to send mail there than to send mail to Australia and other countries which aren't as close.
Generally, two IRC's will cover larger 8x10 photos and envelopes. However, I think it may be possible to get away with 1 IRC

, but it's probably best to send 2 (I know how expensive they are!!).
Good luck with however many you decide to send ... although it's much better if you find a way to get US stamps.
nowandagain [rockon]