There is no sea mail to Australia from the USA, but we do offer sea freight. Because of the high base cost of sea freight it is not significantly cheaper for items under 110 lbs (50 kg). Sea freight uses the same volumetric weight formula as UPS (i.e. 139 cubic inches = 1 lb). Sea freight will normally take 6 to 8 weeks to arrive. Our shipping cost calculator will provide you with an approximate cost of shipping by sea.
One difference with sea freight is it is shipped out of California, not Oregon. This can mean that your items may incur Californian sales tax unlike items shipped by my US partners from Oregon which does not have a state sales tax. My US partners have generally been able to claim back the Californian sales tax, but this can take a couple of month to come through. If they are able to claim the tax back they will refund it to you.