I want to send four small stacking chairs to Australia.  They are too
big to send by mail (length + girth approx. 140"), but not worth
sending by household goods freight movers, which I have found
uneconomic for less than a cubic meter.  They only weigh about 45 lb.

have you considered contacting the airlines or shipyards directly?

Try repacking into 130" total girth, which means [(2xWidth +2xlength)
+ length].  If you can repack the chairs to equal the above, without
compromising the safety of the chairs, then you are able to ship them
through many means.  People tend to think that the USPS (the post
office) is the cheapest way to go when shipping overseas.  This may be
true when sending a letter to Australia for $.60, yet, when it comes
to parcels, courier companies are typically less expensive than the
post office (if you do your hoemwork).  Compare shipping prices with
the following companies, and you will find that most of them will be
cheaper than USPS;

www.ups.com
www.dhl-usa.com
www.homeshipping.com
www.freightquote.com
www.fedex.com

If that does not work, I suggest that you ship the chairs by ocean
freight to australia.  One cubic meter to Australia may cost you less
than 200 usd.  Yet, the advantage is you can ship a lot more than the
chairs for the same price, if you have other things to send.  consider
ocean freight.  Perhaps it's your best way to go.  good luck