I dont believe there are any cheap options - apart from sea mail which you have mentioned - for sending stuff from Australia.

I used to buy the odd thing from the US - some sellers used to send from there via international priority pre paid satchel for smaller items (up to about a kilo ithink) which was very cheap. I dont believe there is any such similar service from Australia - and certainly none fo cheap that would give you any level of seller protection.

If your buyer doesnt like the postage charges imposed by Auspost then dont sell to them.