Just go on united from sfo or lax. you can buy your electronic visa at ticketing for $25 if you are not an australian citizen. Atlas / polar is another option, you'll have to arange the visa through the australian embassy in DC or your nearest city, that could take up to a couple weeks. If you go on united both the lax and sfo a/c stop in SYD and one continues to Melbourne(not sure which one) this could be a problem or more of a hassle jumpseating as you don't won't to get stuck in the transet lounge. As far as taxes, on your return trip they will hit you with taxes. About $75usd.