There's no "by sea" anymore via US Post to Australia, unfortunately. I used to send stuff that way all the time, but it's not an option now - everything goes air, fast or slow.
$90 does seem expensive, but not horribly so. I sent a plastic toy truck to Australia at the end of last year (one of those remote control thingies, about 12 inches long & high) and it cost me about $35. Had no batteries in it and it was pretty light, being all of plastic. It is not cheap to ship stuff here anymore.
US Post does a great job, though. It's fast and the least expensive of the options.
Best thing to do is pack it up, take it the PO to weigh it and get a real number so you can pass that on the buyer.