Lori, I take my packages to Australia to the post office -- the flat rate priority envelope, I believe, is around $12. But I've sent packages of all sizes -- with the understanding that the recipient will pay the exact cost. This is someone I know well though. Australians seem to be aware that the cost of shipping is high from the U.S.
You also will fill out a customs form at the p.o. (there are two, and I'm not sure what the determination is for which one -- size, weight, value?).
You can get a rough idea at usps.com, or even print your postage yourself from there, but I always feel more comfortable taking overseas packages to the p.o.
HTH, friend!